# Diagnostic Cytology: Sample Collection and Interpretation


## Key Takeaways

- Fine needle aspiration (FNA) is the preferred method for solid masses, while imprints are suitable for excised tissues and swabs for mucosal/cutaneous lesions; needle gauge selection (22-25g for most, 18-20g for fibrotic lesions) and smear techniques (squash, blood, line) are critical for preserving cell morphology.
- Romanowsky-type stains (Diff-Quik, Wright-Giemsa) are standard for routine evaluation, with Wright-Giemsa offering superior nuclear detail for hematopoietic/lymphoid assessment; air-dried smears are essential, and unstained smears are preferred for laboratory submission.
- Cytologic interpretation follows a structured framework: assess background (blood, protein, necrosis, inflammation), cellularity (low, moderate, high relative to tissue type), and classify cell populations (discrete round, mesenchymal, epithelial, mixed) to prioritize differential diagnoses.
- Cytology can provide definitive diagnoses for entities like mast cell tumors and lymphoma, but is often supportive for sarcomas; inflammatory patterns are classified by predominant cell type (neutrophilic, granulomatous, etc.), guiding suspicion for infectious agents or foreign bodies.
- Sample quality control is paramount; recollect hemodilute, acellular, or poorly stained samples, and recognize limitations such as inability to assess tissue architecture, invasion, or complete histologic grading, necessitating histopathology for definitive diagnosis and grading.
- Decision points like significant hemodilution, marked necrosis/inflammation, or sampling of cystic structures necessitate reassessment of technique, further sampling, or referral to histopathology to avoid misinterpretation or missed diagnoses.

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Cytology is the microscopic evaluation of individual cells obtained from lesions, fluids, or tissue imprints. It offers a rapid, minimally invasive means of generating a morphologic diagnosis or a prioritized differential list, and it frequently guides the decision to pursue histopathology, culture, or molecular testing. This article provides a procedural framework for sample acquisition, smear preparation, staining, and systematic interpretation across species. It is written for veterinary students and practitioners who seek a reproducible approach to in-house cytologic evaluation and an understanding of when cytology is diagnostically sufficient versus when it must be supplemented by other modalities.

The clinical questions cytology answers are specific: Is this mass inflammatory or neoplastic? If neoplastic, is the population discrete or mesenchymal? Is this effusion a transudate, an exudate, or a neoplastic effusion? Does this lymph node reflect reactive hyperplasia or lymphoma? Cytology cannot provide tissue architecture, invasion status, or complete histologic grading, and it is unreliable for certain tumors with marked desmoplasia or scant exfoliation. Recognizing these limits is as important as mastering the technique itself.

This article covers collection methods including fine needle aspiration, imprint, and swab preparations, smear techniques that preserve cell morphology, staining principles for Romanowsky-type and other stains, and a structured interpretive framework based on cellularity, cell type, and background. Reference intervals for hematologic and biochemical analytes are addressed elsewhere in this series, but the same statistical rigor that underlies those intervals applies to cytologic interpretation: a diagnosis is only as valid as the sample quality and the interpreter's awareness of normal variation.

## At a Glance

| Parameter or Decision | Key Information |
|---|---|
| Sample collection method | Fine needle aspiration (FNA) preferred for solid masses, imprint for excised tissue, swab for mucosal or cutaneous lesions |
| Needle selection | 22 to 25 gauge for most masses, 18 to 20 gauge for markedly cellular or fibrotic lesions, no syringe needed for nonaspiration technique |
| Smear technique | Squash, blood smear, or line smear, choose based on sample viscosity and cellularity |
| Staining | Diff-Quik or Wright-Giemsa for routine evaluation, rapid stains acceptable for in-house use but may distort nuclear detail |
| Cellularity assessment | Low, moderate, or high, compare to expected yield for tissue type |
| Cell population classification | Discrete round cells, mesenchymal cells, epithelial cells, or mixed, this drives differential prioritization |
| Background evaluation | Blood, proteinaceous fluid, necrosis, inflammation, or crystalloid material, each alters interpretation |
| Diagnostic confidence | Cytology is definitive for some entities (mast cell tumor, lymphoma, carcinoma) and supportive only for others (sarcoma, mesenchymal neoplasia) |
| Quality control | Recollect if sample is hemodilute, acellular, or poorly stained, do not overinterpret inadequate samples |

## Principles of Sample Acquisition

The goal of any cytologic collection is a representative, viable, minimally hemorrhagic sample. Cell yield depends on the lesion's cellularity, stromal content, and cohesion. Epithelial tumors often exfoliate readily, whereas mesenchymal tumors such as fibrosarcoma or peripheral nerve sheath tumors may yield few cells because of dense collagenous stroma. Inflammatory lesions with fibrosis behave similarly.

Fine needle aspiration is performed with a 22 to 25 gauge needle attached to a 3 to 12 mL syringe. The mass is immobilized with one hand while the needle is inserted with a rapid, controlled motion. For the aspiration technique, negative pressure is applied after the needle enters the lesion, and the needle is redirected several times within the mass while maintaining suction. Pressure is released before withdrawing the needle to prevent aspiration of the sample into the syringe hub. The nonaspiration technique, in which the needle is moved through the lesion without suction, relies on capillary action and is preferred for vascular or fragile lesions because it reduces hemodilution. Both techniques require multiple passes to maximize yield.

Imprint preparations are made by gently pressing a freshly cut surface of excised tissue onto a clean glass slide. This method preserves cell morphology well but is limited to tissue that is already available. Swab preparations are appropriate for external ear canals, vaginal mucosa, conjunctiva, and draining tracts. The swab is rolled, not rubbed, onto the slide to avoid crushing cells.

## Smear Preparation and Staining

The objective of smear preparation is a monolayer of intact cells with preserved nuclear and cytoplasmic detail. The squash preparation is the most common method for solid tissue aspirates. A drop of sample is placed near the frosted end of one slide, a second slide is lowered onto the first at a right angle, and the sample is allowed to spread by capillary action before the slides are pulled apart. Excessive pressure ruptures cells, insufficient pressure yields a thick, unreadable smear.

The blood smear technique, using a spreader slide at a 30 to 45 degree angle, suits fluid samples and samples with low viscosity. The line smear, in which a drop is dragged across the slide with the edge of another slide, works well for mucoid or viscous samples. For fluid specimens, direct smear and cytocentrifugation are both used, cytocentrifugation concentrates cells but may distort morphology if performed at excessive speed.

Romanowsky-type stains, including Diff-Quik and Wright-Giemsa, are standard. Diff-Quik is rapid and convenient but can overstain cytoplasm and obscure granules. Wright-Giemsa provides superior nuclear detail and is preferred when evaluating hematopoietic or lymphoid populations. Air-dried smears are essential for Romanowsky staining, heat fixation is not used. If a sample must be submitted to a reference laboratory, unstained air-dried smears are generally preferred over stained slides, as the laboratory can apply its own optimal stain.

## The Interpretive Framework

Interpretation begins with a low-power scan to assess overall cellularity, then progresses to high-power evaluation of individual cells. The interpreter should first classify the background: blood, proteinaceous fluid, necrosis, or inflammation. A hemodilute sample may still be diagnostic if intact cells are present, but a sample composed almost entirely of blood should be recollected.

Cellularity is judged against the expected yield for the tissue. A lymph node aspirate should be highly cellular, a fibrotic mass yielding sparse cells is not necessarily inadequate. The next step is classification of the cell population into one of four patterns: discrete round cells, mesenchymal cells, epithelial cells, or mixed. Discrete round cells, such as lymphocytes, plasma cells, mast cells, and histiocytes, are individually distinct and often exfoliate in large numbers. Mesenchymal cells are spindle-shaped or stellate and may form streams or whorls. Epithelial cells appear in sheets, clusters, or acinar arrangements with distinct cell borders. Mixed populations raise the possibility of inflammation with reactive mesenchymal or epithelial cells, or of a tumor with an inflammatory component.

## Diagnostic Patterns and Limitations

Certain cytologic findings are diagnostic. Mast cell tumors show sheets of round cells with metachromatic cytoplasmic granules. Lymphoma yields a monomorphic population of large lymphoid cells with scant cytoplasm and prominent nucleoli. Carcinomas often present as cohesive clusters of epithelial cells with marked anisocytosis and anisokaryosis. In contrast, sarcomas may yield only a few spindle cells, and cytology can support but rarely confirm a specific mesenchymal diagnosis. Histopathology is required for definitive grading and margin assessment.

Inflammatory patterns are classified by the predominant cell type: neutrophilic, pyogranulomatous, granulomatous, eosinophilic, or mixed. Neutrophilic inflammation with intracellular bacteria supports bacterial infection, whereas a predominance of macrophages and multinucleated giant cells suggests a foreign body or fungal etiology. The absence of organizms on cytology does not exclude infection, culture and sensitivity should be pursued when clinical suspicion is high.

Reference values for cytologic features are not standardized across species, and the interpreter must account for species-specific variation in normal cell morphology and background. The same caution applies to the interpretation of effusions, where classification as transudate, modified transudate, or exudate depends on total protein and nucleated cell count, and where neoplastic effusions may require confirmation by immunocytochemistry or flow cytometry. The [Davis-Thompson Foundation veterinary pathology resources](https://www.davisthompsonfoundation.org/) provide case-based material for developing interpretive skill, and the [MSD Veterinary Manual professional edition](https://www.msdvetmanual.com/) offers species-specific guidance on sample collection and common cytologic findings.

## Assessment Sequence for Cytology Samples

The interpretive process begins before the smear reaches the microscope. A complete assessment requires integration of signalment, lesion location, gross appearance, and the cytologic findings. Each element constrains the differential list. A round cell population in a dermal mass in a young dog suggests different entities than the same population in a nasal planum lesion in an older cat.

Start with low magnification (4x or 10x objective) to evaluate overall cellularity, architecture, and the presence of acellular material. Scan the entire smear, including the feathered edge and the thick regions near the drop origin. Cellular detail is often best preserved at the feathered edge, while thick areas may reveal tissue fragments or aggregates lost in thinner regions. Move to high magnification (50x or 100x oil immersion) only after the low-power survey is complete.

Assess cellularity semiquantitatively. A highly cellular smear from a fluid-filled mass supports an exudative or neoplastic process. A paucicellular smear from a firm, fibrous lesion may reflect sampling error, desmoplasia, or a cyst that collapsed during aspiration. Repeat aspiration with a larger needle or different angle when cellularity is unexpectedly low.

Evaluate cell preservation. Nuclear detail, cytoplasmic borders, and granule morphology degrade rapidly after collection. Poorly preserved cells show karyorrhexis, cytoplasmic vacuolation, or smudged chromatin. When preservation is poor, the cytologic diagnosis should be downgraded in confidence and repeat sampling recommended.

## Decision Points That Change the Diagnostic Path

Several findings during assessment should redirect the approach.

**Hemodilution.** Blood contamination dilutes the diagnostic cell population and introduces peripheral blood elements that can be mistaken for pathologic cells. A hemodilute sample from a vascular mass may show only erythrocytes and a few leukocytes. If the lesion is firm or deep, redirect the needle through a different tissue plane or use a smaller gauge needle to reduce capillary trauma.

**Necrosis and inflammation.** Neutrophils, macrophages, and cellular debris can obscure an underlying neoplastic population. When inflammation is marked, the differential list must include both infectious agents and neoplasia with secondary inflammation. Search the smear systematically for infectious organizms before concluding that inflammation is sterile.

**Cystic or fluid-filled lesions.** Aspiration of a cystic structure yields fluid that may be acellular or contain only degenerate cells. The fluid should be submitted for fluid analysis, including total protein and cell count, and the cyst wall should be reaspirated separately. A negative aspirate from the fluid does not exclude a neoplastic cyst wall.

**Multiple lesions.** Aspirate each distinct lesion separately. Label each slide with the lesion location and the needle pass number. Pooling samples from different sites destroys the topographic information needed for staging and surgical planning.

## Cytologic Patterns and Their Differential Lists

The following table organizes the major cytologic patterns and the differentials that should be considered for each. This framework is applied after the cellular population has been characterized.

| Pattern | Cytologic Features | Differential Considerations |
|---|---|---|
| Round cell | Discrete, round cells with distinct cytoplasmic borders | Lymphoma, mast cell tumor, plasma cell tumor, histiocytoma, transmissible venereal tumor |
| Epithelial | Cells in sheets, clusters, or acini with defined cell borders | Carcinoma, adenoma, epithelial hyperplasia, squamous cell carcinoma |
| Mesenchymal | Spindle to stellate cells with indistinct borders, often in streams | Sarcoma, fibroma, granulation tissue, desmoplasia |
| Mixed inflammatory | Neutrophils, macrophages, lymphocytes, plasma cells in variable proportions | Bacterial infection, fungal infection, sterile pyogranuloma, foreign body reaction |
| Suppurative | Predominantly neutrophils, often degenerate | Bacterial infection, sterile neutrophilic inflammation, neoplasia with necrosis |
| Granulomatous | Epithelioid macrophages, multinucleated giant cells, lymphocytes | Fungal infection, mycobacterial infection, foreign body, sterile idiopathic granuloma |
| Hemorrhagic | Erythrocytes with macrophages containing hemosiderin or erythrophagia | Recent or resolving hemorrhage, vascular neoplasia, coagulopathy |
| Cystic | Acellular fluid, cholesterol clefts, foamy macrophages | Epidermal cyst, apocrine cyst, salivary mucocele, abscess |

The pattern approach narrows the differential list but does not replace the need for specific cell identification. A round cell population requires further characterization of nuclear morphology, cytoplasmic granules, and mitotic activity. Mast cell tumors show metachromatic granules on Romanowsky stains, but granule density varies widely between tumors and even within a single smear. Lymphoma typically shows a monomorphic population of large lymphoid cells with scant cytoplasm and visible nucleoli, whereas reactive lymphoid hyperplasia shows a mixed population with small and large lymphocytes.

## Species and Site Modifications

The correct sampling technique and interpretive framework vary with species and anatomic site.

**Bone marrow aspiration** requires a different needle, stylet, and collection technique than soft tissue aspiration. The sample is evaluated for megakaryocyte number, myeloid to erythroid ratio, and the presence of abnormal cell populations. Marrow aspirates are indicated for unexplained cytopenias, suspected hematopoietic neoplasia, and staging of certain malignancies. The iliac crest, proximal humerus, and proximal femur are common sites in dogs and cats. In horses, the sternum and rib are accessible. Rodent bone marrow collection is performed at necropsy or from the femur or tibia under anesthesia.

**Lymph node aspiration** is performed with a small gauge needle and gentle suction. The cytologic pattern distinguishes reactive hyperplasia, lymphoma, and metastatic neoplasia. A reactive node shows a mixed population with increased medium and large lymphocytes and plasma cells. Lymphoma shows a monomorphic population, often with a high mitotic rate. Metastatic carcinoma or sarcoma appears as clusters of epithelial or mesenchymal cells within a background of lymphoid cells.

**Cerebrospinal fluid** collection requires aseptic technique and specialised handling. The sample must be analyzed within 30 to 60 minutes of collection because cells degenerate rapidly. A nucleated cell count and differential are performed, and the fluid is examined for neoplastic cells, infectious agents, and hemorrhage.

**Respiratory samples** obtained by tracheal wash, bronchoalveolar lavage, or transthoracic aspiration require different preparation methods. Lavage fluid is processed by cytocentrifugation to concentrate cells. Transthoracic aspirates of pulmonary masses are prepared as direct smears. The interpretation must account for the normal flora and the expected cell populations at each site.

**Production animals.** In cattle, sheep, and goats, cytology is commonly used for the evaluation of mastitis, joint infections, and cutaneous masses. Sample collection in large animals may require sedation or regional anesthesia. The same interpretive framework applies, but the differential list must include species-specific infectious agents such as *Trueperella pyogenes*, *Corynebacterium pseudotuberculosis*, and *Mycobacterium avium* subspecies *paratuberculosis*.

**Exotic species.** Cytologic interpretation in birds, reptiles, and small mammals follows the same principles, but reference intervals and normal cell morphology differ. Avian erythrocytes are nucleated, and thrombocytes are nucleated cells that can be confused with lymphocytes. Reptilian leukocytes include heterophils, azurophils, and basophils, and their morphology differs from mammalian neutrophils. Reference values for hematologic and biochemical analytes in laboratory rodents are sex-specific, and the same principle applies to cytologic interpretation: normal ranges must be established for the species, strain, age, and sex under evaluation [sex-specific reference intervals in Sprague-Dawley rats](https://pubmed.ncbi.nlm.nih.gov/29261747/).

## Documentation and Reporting

Every cytology report should include the following elements:

- Patient identification, signalment, and relevant history
- Lesion location and gross description
- Sample collection method, needle gauge, and number of passes
- Smear quality assessment, including cellularity and preservation
- Cytologic description of the cell populations present
- Interpretation, expressed as a cytologic diagnosis or a differential list
- Recommendations for additional sampling, culture, or histopathology

The report should distinguish between a definitive cytologic diagnosis and a descriptive interpretation. A definitive diagnosis is possible when the cell population is pathognomonic, such as a mast cell tumor with abundant metachromatic granules or a plasma cell tumor with characteriztic morphology. More often, the report lists differentials ranked by likelihood and recommends histopathology for confirmation.

Reference intervals for cytologic parameters are less standardized than for hematologic and biochemical analytes. The principles of reference interval determination, including the need for a minimum of 120 reference individuals and the use of nonparametric methods, apply to cytologic variables as they do to other laboratory parameters [reference value determination in veterinary clinical pathology](https://pubmed.ncbi.nlm.nih.gov/19737162/). In practice, most cytologic interpretation relies on pattern recognition and published descriptions of disease entities instead of quantitative reference intervals.

Digital images of representative fields should be archived with the report. These images support case review, teaching, and medicolegal documentation. The [Davis-Thompson Foundation veterinary pathology resources](https://www.davisthompsonfoundation.org/) provide case material and teaching collections that illustrate the range of cytologic patterns encountered in practice.

## Complications and Failure Modes

Cytology samples fail for predictable reasons. The most common complication is hemodilution, which occurs when the needle tip enters a vessel or when capillary bleeding floods the aspiration site. Blood-diluted samples show erythrocytes scattered between nucleated cells, and the diagnostic yield drops when erythrocytes outnumber nucleated cells by more than roughly ten to one. Detect hemodilution early by examining the feathered edge first, if the smear is uniformly bloody with sparse nucleated cells, recollect before the lesion is obscured by clot formation.

Exfoliation failure produces samples with too few cells for interpretation. This occurs with densely collagenous masses, such as fibrosarcomas or scar tissue, and with well-differentiated epithelial tumors that maintain strong intercellular adhesion. The smear shows only blood and a few stromal fragments. The discriminating check is to repeat aspiration with a larger gauge needle, use a nonaspiration capillary technique, or sample the lesion periphery where cells are less cohesive.

Cell rupture during smear preparation creates nuclear streaming and cytoplasmic debris that mimics necrosis. This artifact is detected by the presence of intact cells alongside ruptured ones and by the absence of inflammatory cells that would accompany true necrosis. Correct by preparing thinner smears and using less pressure during the squash technique.

Clotting within the needle hub traps cells and produces a sample that appears sparsely cellular despite a grossly adequate aspirate. Discard the first pull if the hub shows clot, and prepare smears immediately after collection. Anticoagulant is not routinely used for fine needle aspiration because it dilutes the sample and can distort nuclear detail.

## Common Errors and Corrective Actions

Less experienced clinicians frequently misclassify mesenchymal cells as epithelial cells. Spindle cells in a reactive fibroblastic response can form sheets that mimic epithelium, particularly in chronic inflammation. The corrective action is to examine cell borders: epithelial cells have distinct, often polygonal borders, while mesenchymal cells show tapered, poorly defined cytoplasmic extensions. When in doubt, report the pattern as spindle cell population and recommend histopathology.

Overinterpretation of reactive mesothelial cells as carcinoma is a recurring error in body cavity fluids. Reactive mesothelium shows marked anisocytosis, binucleation, and prominent nucleoli, all features that overlap with malignancy. The discriminating feature is the presence of cell-to-cell windows, which are gaps between adjacent mesothelial cells, and the absence of true acinar or papillary clusters. If the distinction cannot be made, the sample should be described as atypical mesothelial proliferation and submitted for histopathology or ancillary testing.

Sampling the wrong site produces misleading results. Necrotic centers of large masses yield only cellular debris, and cystic lesions yield fluid that may contain few diagnostic cells. Aspirate the viable rim of a mass and sample multiple sites within large or heterogeneous lesions. For cystic structures, drain the fluid, then reaspirate the residual wall.

A less common but consequential error is confusing extracellular matrix with cytoplasm. Myxoid matrix, collagen, and mucin all stain pale blue to pink and can be mistaken for epithelial cytoplasm. The corrective action is to look for nuclei, matrix does not contain nuclei, and any structure interpreted as a cell must have one.

## Limitations of Current Evidence

The cytologic literature relies heavily on retrospective case series and expert opinion. Prospective studies comparing cytologic diagnosis to histopathologic gold standards are limited, and most published accuracy figures come from single institutions with specific case mixes. Reported sensitivity for malignancy detection in canine and feline masses ranges widely, from approximately 60% to over 95%, depending on tumor type and site. This variability means that a negative cytology result does not exclude neoplasia, and a positive result should be confirmed by histopathology before major therapeutic decisions.

Reference intervals for cytologic features, such as nuclear-to-cytoplasmic ratios or nucleolar dimensions, are not established across species. The statistical rigor applied to hematologic reference intervals, as described in the veterinary clinical pathology literature, has no cytologic equivalent. Expert opinion still differs on the threshold for calling a population atypical versus malignant, and this distinction remains subjective. The Davis-Thompson Foundation maintains teaching collections that illustrate these interpretive boundaries, and review of such material is a practical way to calibrate diagnostic thresholds.

## Escalation and Referral

Refer for histopathology when cytology is nondiagnostic, when the cytologic pattern is atypical but not definitive, or when the result would change a treatment plan that carries significant risk. This includes suspected malignancies where surgical margins or chemotherapy decisions depend on tumor grade, which cytology cannot reliably provide.

Consult a clinical pathologist when the sample contains cells you cannot classify, when inflammation and neoplasia coexist and their relationship is unclear, or when a body cavity fluid shows mixed or atypical populations. Most veterinary diagnostic laboratories offer same-day cytology review by a boarded clinical pathologist, and this service is appropriate for any sample that will guide immediate clinical decisions.

Laboratory involvement is also warranted for quality control. If a slide preparation technique produces repeated artifacts, the laboratory can provide feedback on smear thickness, staining, or collection method. Some laboratories offer training sessions for in-house cytology preparation.

Regulatory reporting is rarely triggered by cytology alone. However, lesions consistent with reportable diseases, such as vesicular lesions in livestock or mass lesions in animals destined for international movement, may require notification. Consult the World Organization for Animal Health terrestrial animal health standards for current reporting obligations, and check regional requirements, as these differ between jurisdictions. The American Veterinary Medical Association provides practice resources on professional obligations regarding zoonotic and reportable conditions.

| Observation | Likely Cause | Discriminating Check |
|---|---|---|
| Uniformly bloody smear, few nucleated cells | Hemodilution | Recollect, compare cellularity to blood volume |
| Sparse cells, only stroma fragments | Exfoliation failure | Repeat with larger needle or nonaspiration technique |
| Nuclear streaming, debris with intact cells | Smear artifact | Prepare thinner smear, reduce squash pressure |
| Sheets of spindle cells mistaken for epithelium | Reactive mesenchyme | Examine cell borders for tapered cytoplasmic extensions |
| Atypical cells with cell-to-cell windows | Reactive mesothelium | Look for acinar clusters, consider pathologist review |
| Debris only, no intact cells | Necrotic center sampled | Aspirate viable rim, sample multiple sites |

## Frequently Asked Questions

### How Should I Prioritize Cytology When My Practice Has Limited Equipment or Budget?

A microscope with a good 40x and 100x oil immersion objective is the only non-negotiable item. If a clinical pathology laboratory is accessible, you can prepare and stain smears in-house, then ship them for evaluation by a boarded clinical pathologist. This preserves diagnostic quality while reducing your interpretation burden. When staining is unavailable, air-dried unstained smears remain evaluable for several days if protected from humidity and insects. Romanowsky-type stains are preferred, but rapid field stains are acceptable for screening. If you cannot obtain a diagnostic sample after two or three attempts, proceed to biopsy or referral instead of repeating low-yield aspirates.

### What Should I Do When I Cannot Obtain a Diagnostic Sample From a Solid Mass?

Reassess needle placement. Cystic or necrotic lesions often yield only fluid or debris, aspirate the solid rim instead of the center. For fibrous or scirrhous masses, use a larger gauge needle, apply stronger suction, or perform a needle coring technique with a stylet. If the lesion is mobile, stabilize it with one hand while aspirating with the other. When repeated attempts yield only blood, consider ultrasound guidance to target the lesion periphery. If the sample remains non-diagnostic, histopathology via biopsy is the appropriate escalation. Document the failed attempts and the final decision to pursue biopsy in the medical record.

### How Does Sample Handling Differ Between Species or Sample Types?

Fluid samples such as effusions, cerebrospinal fluid, and urine require anticoagulant in the collection tube to prevent clot formation, which destroys cell morphology. EDTA is preferred for most fluids because it preserves nuclear detail. Bone marrow aspirates require special handling: expel the sample gently onto a slide and prepare a squash preparation immediately, as marrow clots within seconds. For respiratory samples, transtracheal wash fluid should be processed within 30 minutes to minimize neutrophil degeneration. In avian and reptilian species, nucleated erythrocytes and thrombocytes complicate leukocyte counting, cytology interpretation must account for these differences. Always record the collection site and method on the slide label.

### How Should I Document Cytology Findings in the Medical Record?

Record the signalment, lesion location and description, collection method, needle size, and number of aspirates. Describe the smear quality, including cellularity and the degree of hemodilution or artifact. List the cytologic pattern observed, such as suppurative, granulomatous, or neoplastic, and provide the differential diagnoses in order of likelihood. State whether the sample was diagnostic or non-diagnostic and what follow-up is recommended. If slides are submitted to a reference laboratory, note the submission date and laboratory. This documentation supports clinical decision-making and provides a defensible record if the case is revisited or referred.

### How Do I Explain a Non-Diagnostic Cytology Result to a Client or Referring Veterinarian?

Use clear, non-technical language that conveys uncertainty without undermining confidence. State that the sample did not contain enough cells to make a diagnosis, that this is a known limitation of the technique, and that repeat sampling or biopsy is the next step. Explain that cytology is a screening test, not a definitive diagnostic procedure, and that some lesions require tissue biopsy for a conclusive answer. Provide a timeline for the next step and estimate the cost. If you are the referring veterinarian, communicate directly with the specialist about the sample quality and the specific questions you need answered.

### When Should I Refer a Cytology Sample for Expert Review instead of Interpret It Myself?

Refer when the sample shows an unexpected or ambiguous pattern, when the differential list includes a condition that would change therapy substantially, or when you are considering chemotherapy or amputation. Samples from poorly differentiated round cell tumors, mesenchymal neoplasms, and endocrine organs frequently require expert review. If you are uncertain whether the sample is diagnostic, a second opinion is reasonable. The [Davis-Thompson Foundation](https://www.davisthompsonfoundation.org/) provides educational case material that can help you build interpretive skills, and the [MSD Veterinary Manual](https://www.msdvetmanual.com/) offers species-specific guidance on when cytology is appropriate. Reference laboratory pathologists are also available for consultation on difficult cases.

## Related Clinical & Scientific Guides

* [Hypersensitivity Reactions: Types and Mechanisms](/knowledge/veterinary-medicine/veterinary-pathology-microbiology/hypersensitivity-reactions-types-and-mechanisms)
* [Therapeutic Decision-Making for Respiratory Infections in Cattle](/knowledge/veterinary-medicine/veterinary-pathology-microbiology/therapeutic-decision-making-respiratory-infections-cattle)
* [Monitoring Fluid Therapy in Critically Ill Veterinary Patients](/knowledge/veterinary-medicine/veterinary-pathology-microbiology/monitoring-fluid-therapy-critically-ill-veterinary)


## References and Further Reading

- [Sex-specific reference intervals of hematologic and biochemical analytes in Sprague-Dawley rats using the nonparametric rank percentile method.](https://pubmed.ncbi.nlm.nih.gov/29261747/). 2017.
- [APP mouse models for Alzheimer's disease preclinical studies.](https://pubmed.ncbi.nlm.nih.gov/28768718/). 2017.
- [Reference values: a review.](https://pubmed.ncbi.nlm.nih.gov/19737162/). 2009.
- [Davis-Thompson Foundation Veterinary Pathology Resources](https://www.davisthompsonfoundation.org/). Davis-Thompson Foundation.
- [MSD Veterinary Manual, Professional Edition](https://www.msdvetmanual.com/). MSD Veterinary Manual.
- [American Veterinary Medical Association Practice Resources](https://www.avma.org/resources-tools). American Veterinary Medical Association.
- [WOAH Terrestrial Animal Health Code](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/). WOAH.

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> This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.