# Ultrasound-Guided Fine-Needle Aspiration

## Quick Answer

- Ultrasound guidance for fine-needle aspiration (FNA) improves targeting of nonpalpable or deeply located masses and reduces the risk of sampling necrotic or cystic regions, which directly affects diagnostic yield.
- The key practical step is to confirm the lesion is visible on ultrasound, select an appropriate needle gauge based on the suspected tissue type, and use a continuous visualization technique during needle advancement.
- A critical limitation is that ultrasound guidance does not guarantee an adequate cell sample, and a nondiagnostic result still requires repeat sampling or surgical biopsy for definitive diagnosis.

## Clinical Context and Rationale for Ultrasound-Guided FNA

Fine-needle aspiration is a minimally invasive diagnostic procedure used to collect cellular material from masses, organs, and lymph nodes for cytologic evaluation. The technique has become a standard first-line diagnostic step in veterinary medicine because it is rapid, cost-effective, and associated with fewer complications than surgical biopsy. However, the diagnostic value of FNA depends entirely on whether the needle reaches the intended target and whether the collected sample contains enough intact cells for meaningful interpretation.

Palpation-guided FNA has been the traditional approach for superficial masses. The clinician fixes the mass with one hand and advances the needle into the lesion while maintaining negative pressure. This method works well for large, firm, superficial masses that are easily localized. The limitations of palpation-guided FNA become apparent when the target is small, deeply located, mobile, or surrounded by structures that obscure its boundaries. A mass that is barely palpable or not palpable at all cannot be sampled accurately without imaging assistance.

Ultrasound guidance addresses these limitations by providing real-time visualization of the needle tip within the target lesion. The clinician can confirm that the needle is positioned inside the mass before applying suction or performing capillary sampling. This capability is especially important for masses in the liver, spleen, kidneys, pancreas, and deep lymph nodes, where blind sampling carries a higher risk of hitting adjacent blood vessels or other critical structures. The American Veterinary Medical Association emphasizes the importance of regular veterinary examinations and diagnostic procedures as part of preventive care, and ultrasound-guided FNA is a standard component of that diagnostic approach in many practices.

The decision to use ultrasound guidance should be based on the characteristics of the target lesion and the clinical question being asked. A large, superficial, firm mass that is easily palpated may not require ultrasound guidance. A small, deep, or soft mass that is difficult to localize by palpation is a strong candidate for ultrasound-guided sampling. The same applies to lesions that are suspected to have a cystic component, because ultrasound can help the clinician avoid the fluid-filled center and sample the solid wall where diagnostic cells are more likely to be present.

## Core Principles of Ultrasound-Guided FNA

### Physics and Image Orientation

Ultrasound imaging relies on the transmission of high-frequency sound waves into tissue and the detection of returning echoes. The transducer emits pulses of sound that travel through tissue and reflect at boundaries between structures with different acoustic impedances. The reflected waves return to the transducer and are converted into an image on the screen. The depth of a structure is determined by the time it takes for the echo to return, and the brightness of the image is determined by the strength of the reflection.

The operator must understand the orientation of the ultrasound image relative to the transducer. Most ultrasound machines display a marker on the screen that corresponds to a specific side of the transducer. The image on the screen is a two-dimensional slice of the tissue that lies directly beneath the transducer. When the transducer is oriented with the marker toward the patient's head, the left side of the screen corresponds to the patient's head and the right side to the tail. This orientation must be maintained consistently throughout the procedure to ensure accurate needle placement.

The needle appears on the ultrasound image as a bright linear echo because the metal surface reflects sound waves strongly. The visibility of the needle depends on the angle between the needle and the ultrasound beam. The needle is most visible when it is perpendicular to the beam and becomes less visible as the angle approaches parallel. The clinician must adjust the needle angle or the transducer position to maintain visualization of the needle tip throughout the procedure.

### Needle Selection and Preparation

The choice of needle gauge depends on the suspected tissue type and the expected cellularity of the target. A 22-gauge needle is a common choice for most soft tissue masses and lymph nodes. A 25-gauge needle may be used for highly vascular organs such as the liver or for small lesions where bleeding is a concern. A 20-gauge needle may be used for firm, fibrous masses that are difficult to aspirate with a smaller needle. The needle should be long enough to reach the target from the skin surface, but not so long that the clinician loses fine control over the needle tip.

The needle is typically attached to a syringe for aspiration. A 6-mL or 12-mL syringe is commonly used. The syringe provides the negative pressure that draws cells into the needle. The amount of suction applied and the number of passes through the lesion are important variables that affect sample quality. Excessive suction can cause the cells to be disrupted or the sample to be diluted with blood. Insufficient suction may result in a sample that is too sparse for cytologic interpretation.

The needle and syringe should be assembled before the procedure begins. The clinician should confirm that the needle is firmly attached to the syringe and that the syringe plunger moves smoothly. The ultrasound gel should be applied to the skin over the target area, and the transducer should be positioned to provide a clear view of the lesion. The skin over the needle insertion site should be clipped and prepared aseptically, even though FNA is a minimally invasive procedure.

### Patient Preparation and Positioning

Patient preparation is an important component of ultrasound-guided FNA. The patient should be positioned in a way that provides the best access to the target lesion and the most comfortable position for the clinician. The area over the target should be clipped to remove hair, because hair traps air and interferes with ultrasound transmission. The skin should be cleaned with an antiseptic solution to reduce the risk of infection at the needle insertion site.

The patient may be sedated or anesthetized depending on the location of the lesion and the temperament of the patient. A cooperative patient with a superficial mass may not require sedation. A patient with a deep abdominal mass or a patient that is anxious or painful will benefit from sedation or anesthesia. The decision to sedate should be made in consultation with the attending veterinarian, and the level of sedation should be appropriate for the procedure and the patient's health status.

The ultrasound transducer should be covered with a sterile sleeve or a sterile glove to maintain a sterile field at the needle insertion site. The sterile gel should be applied to the transducer or the skin to provide acoustic coupling. The clinician should position the transducer so that the target lesion is clearly visible in the center of the image, and the needle insertion site should be chosen to allow the needle to reach the target without passing through major vessels or other critical structures.

## Practical Workflow for Ultrasound-Guided FNA

### Step 1: Confirm the Target and Plan the Approach

The first step is to perform a complete ultrasound examination of the region of interest. The clinician should identify the target lesion and characterize its size, shape, echogenicity, and vascularity. The lesion should be measured in two or three dimensions, and the depth from the skin surface should be recorded. The clinician should also identify the surrounding structures, including blood vessels, nerves, and other organs, to plan a needle path that avoids these structures.

The approach should be planned to minimize the distance between the skin and the target and to avoid passing through major vessels. The needle should be inserted at an angle that allows the entire needle shaft to be visualized in the ultrasound image. The angle of the needle relative to the transducer is a key factor in maintaining visualization. The needle should be advanced slowly and continuously while the clinician watches the needle tip on the screen.

### Step 2: Position the Transducer and Insert the Needle

The transducer should be positioned over the target lesion, and the image should be optimized to show the lesion clearly. The gain and depth settings should be adjusted to provide the best image quality. The needle insertion site should be marked on the skin, and the skin should be prepared aseptically.

The needle should be inserted through the skin at the marked site and advanced toward the target. The needle should be advanced in a single plane that is aligned with the ultrasound beam. The clinician should watch the needle tip on the screen and adjust the needle angle as needed to keep the tip in view. The needle should be advanced until the tip is clearly visible within the target lesion.

### Step 3: Sample the Lesion

Once the needle tip is confirmed to be within the lesion, the sampling technique can be performed. The two main techniques are capillary sampling and suction-assisted sampling. Capillary sampling involves moving the needle back and forth within the lesion without applying suction. The cells enter the needle by capillary action. Suction-assisted sampling involves applying negative pressure with the syringe while the needle is moved back and forth within the lesion.

The number of passes and the duration of sampling depend on the tissue type and the cellularity of the lesion. The clinician should be careful not to apply excessive suction or to move the needle too aggressively, because this can cause the sample to be diluted with blood or the cells to be destroyed. The needle should be withdrawn from the lesion and the patient after the sample is collected.

### Step 4: Prepare the Smear and Evaluate the Sample

The sample should be expelled from the needle onto a glass slide. The sample should be spread into a thin smear using a second slide or a coverslip. The smear should be air-dried and stained with a Romanowsky-type stain, such as Diff-Quik, for immediate evaluation. The clinician should evaluate the smear under a microscope to assess the cellularity and the quality of the sample.

The sample is considered adequate if it contains a sufficient number of intact cells that are representative of the lesion. A sample that contains only blood or that has a very low cellularity may be nondiagnostic. The clinician should decide whether to repeat the FNA or to proceed with a surgical biopsy based on the quality of the sample and the clinical context.

### Step 5: Post-Procedure Care and Monitoring

After the FNA is complete, the needle insertion site should be cleaned and the patient should be monitored for any signs of bleeding or other complications. The patient should be observed for a short period after the procedure to ensure that there is no excessive bleeding or swelling at the site. The patient should be allowed to recover from sedation or anesthesia, and the owner should be given instructions for monitoring the site at home.

The sample should be submitted to a clinical pathology laboratory for cytologic interpretation. The clinician should provide the laboratory with a complete history, including the signalment of the patient, the location of the lesion, and the ultrasound findings. The laboratory report should be interpreted in the context of the clinical findings and the ultrasound images.

## At a Glance

| Clinical Scenario | Recommended Approach | Key Consideration |
| --- | --- | --- |
| Superficial, firm, palpable mass | Palpation-guided FNA may be sufficient | Confirm the mass is fixed and accessible before sampling |
| Deep abdominal mass (liver, spleen, kidney) | Ultrasound-guided FNA is strongly recommended | Use real-time visualization to avoid vessels and confirm needle tip placement |
| Small or nonpalpable lesion | Ultrasound-guided FNA is required | The needle tip must be confirmed within the lesion before sampling |
| Cystic or necrotic mass | Ultrasound-guided FNA with targeted sampling of the solid wall | Avoid the fluid-filled center to improve diagnostic yield |
| Coagulopathic patient | Ultrasound-guided FNA with caution and minimal passes | The risk of bleeding is increased, and the procedure should be discussed with the attending veterinarian |

## Options and Tradeoffs in Sampling Technique

### Capillary Sampling Versus Suction-Assisted Sampling

The choice between capillary sampling and suction-assisted sampling depends on the tissue type and the cellularity of the lesion. Capillary sampling is a gentler technique that is less likely to cause bleeding or cell destruction. It is often preferred for vascular organs such as the liver and the spleen, where suction can cause significant hemorrhage. Capillary sampling is also useful for lesions that are highly cellular, because the cells are easily drawn into the needle by capillary action.

Suction-assisted sampling is more aggressive and is useful for lesions that are fibrous or that have a low cellularity. The negative pressure helps to pull cells into the needle, but it also increases the risk of blood contamination and cell destruction. The suction should be applied only after the needle tip is confirmed to be within the lesion, and the suction should be released before the needle is withdrawn from the lesion.

### Needle Gauge and Sample Quality

The needle gauge has a direct effect on the sample quality. A smaller needle, such as a 25-gauge, produces a sample with less blood contamination but may not be able to aspirate cells from a fibrous lesion. A larger needle, such as a 20-gauge, can aspirate more cells but may cause more bleeding and may be more painful for the patient. The clinician should choose the needle gauge based on the suspected tissue type and the cellularity of the lesion.

The needle length should be sufficient to reach the target from the skin surface. A needle that is too short will not reach the lesion, and a needle that is too long will be difficult to control. The clinician should measure the depth of the lesion on the ultrasound image and choose a needle that is long enough to reach the target with a margin of safety.

### Number of Passes and Sample Adequacy

The number of passes performed during the FNA affects the sample adequacy. A single pass may not be sufficient to obtain a representative sample, particularly if the lesion is heterogeneous or if the cellularity is low. Multiple passes can increase the yield, but they also increase the risk of bleeding and the discomfort for the patient. The clinician should balance the need for an adequate sample against the risk of complications.

The sample should be evaluated immediately after each pass to determine whether it is adequate. If the sample is inadequate, the clinician should decide whether to repeat the FNA or to proceed to a surgical biopsy. The decision should be based on the clinical context and the risk of the procedure.

## Records and Measurements

### Documenting the Procedure

The ultrasound-guided FNA procedure should be documented in the patient's medical record. The documentation should include the indication for the procedure, the ultrasound findings, the needle gauge and length, the sampling technique, the number of passes, and the quality of the sample. The ultrasound images should be saved and stored with the medical record.

The documentation should also include the patient's signalment, the clinical history, and the results of any previous diagnostic tests. The clinician should note any complications that occurred during the procedure and the outcome of the procedure. The documentation should be complete enough to allow another clinician to understand the procedure and the results.

### Recording the Sample Quality

The sample quality should be recorded in the medical record. The sample should be described in terms of the cellularity, the presence of blood, and the presence of any artifacts. The sample should be classified as adequate or inadequate for cytologic interpretation. The clinician should note the number of passes that were performed and the technique that was used.

The sample quality should be correlated with the ultrasound findings. A lesion that appears hypoechoic on ultrasound may be more likely to be necrotic or cystic, and the sample may be less cellular. A lesion that appears hyperechoic may be more likely to be fibrous, and the sample may be more difficult to obtain. The clinician should use the ultrasound findings to guide the sampling technique and to interpret the sample quality.

### Monitoring the Patient

The patient should be monitored after the procedure for any signs of complications. The clinician should observe the patient for bleeding, swelling, or pain at the needle insertion site. The patient should be monitored for any signs of systemic complications, such as a drop in blood pressure or a change in heart rate. The patient should be observed for a period of time after the procedure, and the owner should be instructed to report any signs of complications.

The patient's vital signs should be recorded before and after the procedure. The clinician should note any changes in the patient's condition and should take appropriate action if the patient shows signs of distress. The patient should be allowed to recover from the sedation or anesthesia, and the owner should be provided with instructions for the follow-up care.

## Common Failure Patterns

### Nondiagnostic Samples

The most common failure pattern in ultrasound-guided FNA is a nondiagnostic sample. A nondiagnostic sample is one that contains too few cells or that contains only blood or other noncellular material. The causes of a nondiagnostic sample include a needle that is not placed within the lesion, a lesion that is necrotic or cystic, and a sampling technique that is too gentle or too aggressive.

The clinician should evaluate the sample immediately after each pass and decide whether to repeat the pass. If the sample is nondiagnostic, the clinician should consider the possibility that the needle was not within the lesion. The needle tip should be confirmed within the lesion before the sample is collected. The clinician should also consider the possibility that the lesion is necrotic or cystic, and the needle should be directed to the solid wall of the lesion.

### Blood Contamination

Blood contamination is a common problem in FNA, particularly in highly vascular organs such as the liver and the spleen. Blood contamination can dilute the sample and make it difficult to interpret the cytology. The clinician should use a gentle sampling technique and a smaller needle to reduce the risk of blood contamination. The clinician should also avoid the needle through the major vessels and should apply the suction only after the needle tip is within the lesion.

The blood contamination can be reduced by using the capillary sampling technique and by moving the needle gently within the lesion. The clinician should also consider the possibility that the lesion is a vascular structure, such as a hemangioma, and the sample should be interpreted with caution.

### Needle Tip Visualization

The needle tip may be difficult to visualize on the ultrasound image, particularly if the needle is at a steep angle to the ultrasound beam. The clinician should adjust the needle angle or the transducer position to maintain the needle tip in view. The needle tip should be confirmed within the lesion before the sample is collected.

The needle tip can be visualized more easily if the needle is aligned with the ultrasound beam. The clinician should use a needle guide or a freehand technique to maintain the needle in the plane of the ultrasound beam. The needle tip should be confirmed within the lesion before the sample is collected.

### The Lesion Is Not Visible on Ultrasound

Some lesions are not visible on the ultrasound image, particularly if the lesion is very small or if the lesion has an echogenicity that is similar to the surrounding tissue. The clinician should use the clinical findings and the palpation to guide the needle placement. The clinician should also consider the possibility of using a different imaging modality, such as computed tomography, to guide the needle placement.

The lesion may be visible on the ultrasound image but may be difficult to distinguish from the surrounding tissue. The clinician should use the ultrasound to identify the lesion and to confirm the needle tip within the lesion. The clinician should also consider the possibility of using the ultrasound to guide the needle placement in a different plane.

## Welfare and Safety Context

### The Patient Welfare

The patient welfare is an important consideration in ultrasound-guided FNA. The procedure is minimally invasive, but it can still cause discomfort and pain. The clinician should use the appropriate level of sedation or anesthesia to minimize the patient's discomfort. The clinician should also use a gentle technique and a small needle to reduce the pain and the risk of complications.

The patient should be monitored for any signs of pain or distress during the procedure. The clinician should be prepared to stop the procedure if the patient shows signs of distress. The patient should be provided with the appropriate analgesia after the procedure, and the owner should be instructed to monitor the patient for any signs of pain or complications.

### The Safety of the Clinician

The safety of the clinician is also an important consideration. The clinician should be trained in the technique of ultrasound-guided FNA and should be familiar with the ultrasound equipment. The clinician should use the appropriate personal protective equipment, including gloves and a gown, to prevent the exposure to the blood and the other body fluids.

The clinician should also be aware of the risk of the needle stick injury and should use the appropriate precautions to prevent the injury. The needle should be disposed of in a sharps container, and the clinician should not recap the needle. The clinician should also be aware of the risk of the exposure to the zoonotic diseases and should use the appropriate precautions.

### The Regulatory Context

The ultrasound-guided FNA is a diagnostic procedure that is performed by the veterinarian. The procedure is regulated by the state and the federal laws, and the veterinarian should be licensed to perform the procedure. The veterinarian should also be familiar with the guidelines and the standards of the practice, such as the guidelines from the American Animal Hospital Association and the World Small Animal Veterinary Association.

The veterinarian should also be familiar with the animal health and welfare regulations, and the veterinarian should ensure that the procedure is performed in a manner that is consistent with the animal welfare standards. The veterinarian should also be familiar with the reporting requirements for the animal health and welfare, and the veterinarian should report any suspected cases of the animal disease to the appropriate authorities.

## Limitations and Escalation Criteria

### Limitations of Ultrasound-Guided FNA

Ultrasound-guided FNA has several limitations. The procedure is a sampling technique, and the sample may not be representative of the entire lesion. The lesion may be heterogeneous, and the sample may be taken from a region that is not representative of the lesion as a whole. The sample may also be nondiagnostic, and the clinician may need to repeat the procedure or to proceed with a surgical biopsy.

The ultrasound-guided FNA is also limited by the skill of the clinician and the quality of the ultrasound equipment. The clinician should be trained in the technique and should be familiar with the ultrasound equipment. The ultrasound equipment should be of good quality and should be maintained properly.

### Escalation Criteria

The clinician should escalate the procedure to a surgical biopsy if the FNA is nondiagnostic or if the cytologic findings are inconclusive. The clinician should also escalate the procedure if the lesion is not accessible by the FNA or if the lesion is suspected to be a malignant tumor that requires a histologic diagnosis.

The clinician should also escalate the procedure if the patient has a complication, such as a significant bleeding or a pneumothorax. The clinician should be prepared to manage the complications and to refer the patient to a specialist if necessary.

The clinician should also escalate the procedure if the patient is not a candidate for the FNA, such as a patient with a coagulopathy or a patient that is not stable. The clinician should consider the risk of the procedure and the benefit of the procedure, and the clinician should make the decision based on the clinical context.

## Decision Framework for Choosing Between Palpation-Guided and Ultrasound-Guided FNA

A structured decision framework helps clinicians move from a subjective preference to a consistent, defensible choice between palpation-guided and ultrasound-guided FNA. The framework below uses three assessment domains that can be completed in under two minutes during the initial physical examination and ultrasound survey. Each domain produces a score that directs the clinician toward the appropriate sampling method.

### Domain 1: Lesion Accessibility Assessment

The first domain evaluates whether the lesion can be reliably fixed in position and localized by palpation alone. A lesion that is freely movable under the skin, located in a region with thick overlying tissue, or positioned near mobile structures such as the axilla or inguinal region will be difficult to stabilize during needle insertion. The clinician should attempt to immobilize the lesion between two fingers and assess whether the lesion remains fixed during gentle pressure. If the lesion slips away from the fingers or cannot be held in a stable position, ultrasound guidance is indicated.

The depth of the lesion is the second accessibility factor. A lesion that lies deeper than 2 centimeters from the skin surface in a small animal patient is generally difficult to sample accurately by palpation because the clinician cannot feel the needle tip within the lesion. The clinician should measure the depth from the skin surface to the near edge of the lesion using the ultrasound calipers. Lesions deeper than 2 centimeters should be sampled under ultrasound guidance.

The third accessibility factor is the relationship of the lesion to adjacent structures. A lesion that is adjacent to a major vessel, nerve, or body cavity requires ultrasound guidance to avoid inadvertent puncture. The clinician should identify the lesion on ultrasound and trace the surrounding anatomy before deciding on the sampling approach.

### Domain 2: Lesion Characteristics on Ultrasound

The second domain uses the ultrasound appearance of the lesion to predict the likelihood of obtaining a diagnostic sample. The echogenicity of the lesion relative to the surrounding tissue is the first consideration. A lesion that is isoechoic to the surrounding tissue will be difficult to distinguish on the ultrasound image, and the needle tip may be placed outside the lesion without the clinician recognizing the error. A hypoechoic or hyperechoic lesion is easier to target because the boundary is more distinct.

The internal architecture of the lesion is the second consideration. A lesion with a central anechoic region, which indicates a cystic or necrotic component, requires targeted sampling of the solid wall. The clinician must be able to visualize the needle tip within the solid portion of the lesion to obtain a diagnostic sample. A lesion that is uniformly hypoechoic may be more likely to yield a cellular sample, while a lesion with mixed echogenicity may contain areas of necrosis that produce a nondiagnostic sample.

The vascularity of the lesion should be assessed using color Doppler if available. A lesion with prominent vascular flow will have a higher risk of blood contamination during sampling. The clinician should plan the needle path to avoid the largest vessels and should consider using a smaller gauge needle or the capillary sampling technique for vascular lesions.

### Domain 3: Patient and Procedural Factors

The third domain considers the patient's temperament, the presence of coagulopathy, and the need for sedation. A patient that is anxious, painful, or uncooperative will not remain still during the procedure, and the needle tip may move out of the lesion during sampling. Ultrasound guidance is recommended for these patients because the clinician can confirm the needle tip position before and during the sampling.

A patient with a known coagulopathy or a patient receiving anticoagulant therapy has an increased risk of bleeding after FNA. The clinician should use the smallest needle gauge that will obtain an adequate sample and should limit the number of passes. The ultrasound-guided approach allows the clinician to avoid major vessels and to monitor the needle tip during the procedure, which reduces the risk of vascular injury.

The final factor is the clinical urgency of the diagnosis. If the cytologic result will change the immediate treatment plan, the clinician should use the approach that has the highest likelihood of producing a diagnostic sample. Ultrasound guidance is the preferred approach when the diagnosis will determine whether the patient undergoes surgery, receives chemotherapy, or is managed medically.

### Applying the Framework in Practice

The framework can be applied using a simple scoring system. Each domain is scored as 0 or 1. A score of 0 indicates that the palpation-guided approach is acceptable. A score of 1 indicates that ultrasound guidance is recommended. The total score ranges from 0 to 3. A total score of 0 or 1 supports palpation-guided FNA. A total score of 2 or 3 supports ultrasound-guided FNA.

The clinician should record the score in the medical record along with the reason for the decision. This documentation provides a consistent basis for the choice of procedure and allows the clinician to review the decision if the sample is nondiagnostic. The framework also provides a basis for training new clinicians in the decision-making process.

### Comparison of Palpation-Guided and Ultrasound-Guided FNA

The table below summarizes the key differences between the two approaches across the decision framework domains.

| Decision Factor | Palpation-Guided FNA | Ultrasound-Guided FNA |
| --- | --- | --- |
| Lesion depth | Limited to superficial lesions within 2 cm of the skin | Suitable for lesions at any depth |
| Lesion mobility | Requires a fixed lesion that can be stabilized | Suitable for mobile lesions |
| Lesion visibility | No imaging confirmation of needle tip | Real-time confirmation of needle tip |
| Cystic or necrotic component | Cannot identify internal structure | Can target the solid wall |
| Vascularity assessment | Not available | Color Doppler can assess vascular flow |
| Patient movement | Requires a cooperative patient | Can be performed with sedation or anesthesia |
| Coagulopathy risk | Higher risk of bleeding | Lower risk with careful needle placement |
| Sample adequacy | Depends on palpation accuracy | Depends on needle tip placement and sampling technique |

### Implementing the Framework in Practice

The framework should be used during the initial ultrasound examination before the FNA is performed. The clinician should complete the three domain assessments and record the scores in the medical record. The framework should be applied consistently to all FNA procedures, including those that appear straightforward on palpation.

The clinician should review the framework after each procedure to identify any patterns in the decision-making. If the clinician consistently obtains nondiagnostic samples with palpation-guided FNA for lesions that score 1 in any domain, the threshold for ultrasound guidance should be lowered. The framework should be adjusted based on the clinician's experience and the outcomes of the procedures.

The framework should also be used to guide the discussion with the owner. The owner should be informed of the reason for the choice of the sampling approach and the expected diagnostic yield. The owner should also be informed of the risk of a nondiagnostic sample and the potential need for a repeat procedure or a surgical biopsy.

### Limitations of the Framework

The framework is a clinical decision tool and does not replace the clinician's judgment. The framework does not account for the specific anatomy of the patient or the experience of the clinician. A clinician with extensive experience in palpation-guided FNA may be able to obtain diagnostic samples from lesions that score 1 in one domain. The framework should be used as a starting point for the decision and should be adjusted based on the clinical context.

The framework also does not address the cost of the procedure. Ultrasound-guided FNA is more expensive than palpation-guided FNA because it requires the use of the ultrasound equipment and the time of the clinician. The clinician should discuss the cost with the owner and should consider the financial constraints of the owner when making the decision.

The framework should be reviewed and updated as new evidence becomes available. The clinician should stay current with the veterinary literature and should adjust the framework based on the new evidence. The framework should be used in conjunction with the clinical guidelines from the American Animal Hospital Association and the World Small Animal Veterinary Association.

## Frequently Asked Questions

### What is the difference between ultrasound-guided FNA and palpation-guided FNA?

Ultrasound-guided FNA uses real-time imaging to confirm the needle tip is within the target lesion before sampling. Palpation-guided FNA relies on the clinician's ability to feel the mass and guide the needle by touch. Ultrasound guidance is preferred for deep, small, or nonpalpable lesions because it reduces the risk of sampling the wrong tissue or missing the target.

### How do I know if a lesion is suitable for ultrasound-guided FNA?

A lesion is suitable for ultrasound-guided FNA if it is visible on the ultrasound image and if the needle can be advanced to the lesion without passing through major vessels or other structures. The lesion should be large enough to allow the needle tip to be placed within the lesion, and the lesion should be accessible from the skin surface.

### What needle gauge should I use for ultrasound-guided FNA?

The needle gauge depends on the suspected tissue type and the cellularity of the lesion. A 22-gauge needle is a common choice for most soft tissue masses and lymph nodes. A 25-gauge needle may be used for smaller lesions or for highly vascular organs. A 20-gauge needle may be used for firm masses that are difficult to aspirate with a smaller needle.

### How many passes should I perform during an ultrasound-guided FNA?

The number of passes depends on the cellularity of the lesion and the quality of the sample. A single pass may be sufficient for a cellular lesion, but multiple passes may be needed for a lesion with a low cellularity. The clinician should evaluate the sample after each pass and decide whether to repeat the pass.

### What should I do if the FNA sample is nondiagnostic?

If the FNA sample is nondiagnostic, the clinician should consider the possibility that the needle was not within the lesion or that the lesion is necrotic or fibrous. The clinician should repeat the FNA with a different technique or a different needle gauge. If the sample is still nondiagnostic, the clinician should consider a surgical biopsy.

### What are the complications of ultrasound-guided FNA?

The complications of ultrasound-guided FNA include bleeding, infection, and the pneumothorax. The risk of bleeding is increased in patients with a coagulopathy or in highly vascular organs. The clinician should be prepared to manage the complications and should monitor the patient after the procedure.

### When should I refer a patient for a surgical biopsy instead of an FNA?

A surgical biopsy should be considered if the FNA is nondiagnostic, if the cytology is inconclusive, or if the lesion is a risk of malignancy that requires a histologic diagnosis. A surgical biopsy should also be considered if the lesion is not accessible by ultrasound or if the patient is not a candidate for the FNA.

### How should I document the ultrasound-guided FNA procedure?

The procedure should be documented in the patient's medical record, including the indication, the ultrasound findings, the needle gauge, the sampling technique, the number of passes, and the quality of the sample. The ultrasound images should be saved and the sample should be submitted to the laboratory for cytologic interpretation.

## Related Veterinary Guides

- [Ultrasound-Guided Fine Needle Aspiration in Small Animals: Techniques and Safety](/knowledge/veterinary-medicine/diagnostic-imaging/ultrasound-guided-fine-needle-aspiration-small-animals-techniques-safety)
- [Diagnostic Cytology: Sample Collection and Interpretation](/knowledge/veterinary-medicine/veterinary-pathology-microbiology/diagnostic-cytology-sample-collection-and-interpretation)
- [Veterinary Radiology and Diagnostic Imaging for the NAVLE](/knowledge/veterinary-medicine/navle-exam-prep/veterinary-radiology-diagnostic-imaging-navle)
- [Veterinary Trauma: Secondary Survey and Diagnostic Imaging](/knowledge/veterinary-medicine/emergency-critical-care/veterinary-trauma-secondary-survey-diagnostic-imaging)
- [Using Practice Questions to Improve Diagnostic Accuracy](/knowledge/veterinary-medicine/navle-exam-prep/using-practice-questions-to-improve-diagnostic-accuracy)

## References and Further Reading

- [Pet Care](https://www.avma.org/resources-tools/pet-owners). American Veterinary Medical Association.
- [AAHA Guidelines](https://www.aaha.org/resources). American Animal Hospital Association.
- [Global Guidelines](https://wsava.org/global-guidelines). World Small Animal Veterinary Association.
- [Merck Veterinary Manual](https://www.merckvetmanual.com/). Merck Veterinary Manual.
- [Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/). Cornell University.
- [Animal Health and Welfare](https://www.woah.org/en/what-we-do/animal-health-and-welfare). World Organisation for Animal Health.
- [Colonoscopic Ultrasound-Guided Fine-Needle Aspiration Using a Curvilinear Array Transducer: A Single-Center Retrospective Cohort Study.](https://pubmed.ncbi.nlm.nih.gov/34840298). Diseases of the colon and rectum, 2022.
- [Diagnostic utility of ultrasound-guided fine-needle aspiration and needle-core biopsy sampling of canine splenic masses.](https://pubmed.ncbi.nlm.nih.gov/37790262). Canadian journal of veterinary research = Revue canadienne de recherche veterinaire, 2023.
- [Hepatobiliary Neoplasia.](https://pubmed.ncbi.nlm.nih.gov/28063743). The Veterinary clinics of North America. Small animal practice, 2017.
- [Ultrasound-guided fine-needle aspiration biopsy of bone lesions: a preliminary report.](https://pubmed.ncbi.nlm.nih.gov/10024000). Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association, 1999.
- [Effect of needle gauge on pain and specimen quality of ultrasound-guided fine needle sampling without aspiration of the canine spleen.](https://pubmed.ncbi.nlm.nih.gov/37461325). Veterinary radiology & ultrasound : the official journal of the American College of Veterinary Radiology and the International Veterinary Radiology Association, 2023.

> This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.