# Canine and Feline Acid-Base Disorders: Interpretation and Management ## Key Takeaways - Accurate acid-base interpretation in dogs and cats requires a systematic four-step approach: verify sample quality, identify the primary disturbance (pH, PaCO2, or bicarbonate), calculate expected compensatory responses, and determine if mixed disorders exist. Arterial samples are essential for assessing oxygenation and true respiratory status, while venous samples primarily reflect metabolic status. - Metabolic acidosis is differentiated by the anion gap: a high anion gap indicates accumulation of unmeasured anions (e.g., lactate, ketoacids, uremic anions, ethylene glycol metabolites), while a normal anion gap suggests bicarbonate loss (e.g., diarrhea, renal tubular acidosis) with reciprocal chloride retention. - Expected respiratory compensation for metabolic acidosis in dogs and cats can be estimated using the formula: Expected PaCO2 ≈ 1.5 x HCO3⁻ + 8 (± 2 mmHg). Deviations from this indicate a concurrent respiratory disorder. - Mixed acid-base disorders are identified when measured compensation deviates significantly from expected values, often seen in critically ill patients with conditions like sepsis or cardiopulmonary arrest, requiring identification and treatment of each component. - Species differences are critical: feline kidneys have lower bicarbonate reabsorptive capacity, making them more susceptible to metabolic acidosis and less able to compensate robustly compared to dogs. - The Stewart approach, focusing on strong ion difference, total weak acid concentration, and PaCO2, can clarify complex disorders, particularly in hypoalbuminemic or critically ill patients where the traditional bicarbonate-centered model may be ambiguous. --- Acid-base disorders are among the most common metabolic derangements encountered in canine and feline practice, yet their interpretation is frequently reduced to pattern recognition without a disciplined framework. This article provides a systematic approach to blood gas analysis, classification of primary disturbances, assessment of compensatory responses, and the therapeutic implications that follow from accurate diagnosis. It is written for practicing veterinarians who seek a reproducible method for interpreting venous and arterial blood gas data in dogs and cats. The clinical question this article answers is direct: given a set of blood gas values, what is the primary disorder, is compensation appropriate, and what does the answer mean for patient management? The approach rests on four sequential steps: verify sample quality, identify the primary disturbance, calculate the expected compensatory response, and determine whether a mixed disorder exists. Each step is developed in detail, with species-specific considerations where canine and feline physiology diverge. ## At a Glance | Parameter | Clinical Decision Point | |---|---| | Sample type | Venous pH and bicarbonate reflect metabolic status, arterial samples are required for assessing oxygenation and true respiratory acid-base status | | Primary disturbance | Identify whether pH, PaCO2, or bicarbonate is the initiating abnormality before considering compensation | | Anion gap | Calculated as (Na+ + K+) minus (Cl- + HCO3-), normal range varies by laboratory and analyzer | | Compensation rule, metabolic acidosis | Expected PaCO2 approximates 1.5 x HCO3- + 8, plus or minus 2, in dogs and cats | | Compensation rule, metabolic alkalosis | Expected PaCO2 increases by 0.7 mm Hg per 1 mEq/L rise in bicarbonate | | Compensation rule, respiratory disorders | Metabolic compensation develops over 12 to 72 hours, acute and chronic responses differ | | Mixed disorders | Present when measured compensation deviates from expected values by more than the stated tolerance | | Strong ion approach | Stewart's physicochemical method can clarify disorders when the traditional bicarbonate approach is ambiguous | ## Physiologic Foundations of Acid-Base Balance The body maintains extracellular pH within a narrow range through three integrated systems: extracellular and intracellular buffers, alveolar ventilation, and renal acid excretion. The bicarbonate buffer system is the dominant extracellular buffer and is described by the Henderson-Hasselbalch equation, in which pH is determined by the ratio of bicarbonate to dissolved carbon dioxide instead of by absolute concentrations of either component. This relationship explains why respiratory and metabolic disturbances are inseparable in practice: a change in PaCO2 alters pH directly, while a change in bicarbonate alters pH through the same ratio. Carbon dioxide is produced continuously by oxidative metabolism and eliminated by ventilation. Minute ventilation therefore determines PaCO2, and any disorder that depresses or stimulates ventilation will shift acid-base status within seconds to minutes. The kidneys regulate bicarbonate concentration through proximal tubular reabsorption and distal tubular regeneration, processes that respond over hours to days. This temporal disparity underpins the expected compensatory responses that distinguish simple from mixed disorders. ### The Role of the Anion Gap The anion gap is a calculated value that estimates the concentration of unmeasured anions in plasma. It is derived from the difference between measured cations and measured anions, most commonly (Na+ + K+) minus (Cl- + HCO3-). An elevated anion gap indicates accumulation of organic acids such as lactate, ketoacids, or uremic anions, whereas a normal anion gap metabolic acidosis implies bicarbonate loss with reciprocal chloride retention. The reference interval varies with the analyzer and the inclusion of potassium, so each practice should establish its own range from the laboratory in use. ### Species Differences in Acid-Base Regulation Cats and dogs differ in several clinically relevant aspects of acid-base physiology. Feline kidneys have a lower maximum bicarbonate reabsorptive capacity than canine kidneys, which makes cats more susceptible to metabolic acidosis from renal disease and less able to generate a robust compensatory response. Cats also have a higher basal protein turnover and are obligate carnivores, which influences their acid load from dietary sulfur-containing amino acids. These differences matter most when interpreting chronic kidney disease and when predicting whether a given patient will mount an expected compensation. ## The Stewart Approach to Acid-Base Interpretation The traditional bicarbonate-centered model has limitations, particularly in patients with hypoalbuminemia, dysnatremia, or complex mixed disorders. The Stewart approach, also called strong ion difference theory, reframes acid-base balance around three independent variables: the strong ion difference, total concentration of weak acids (principally albumin and phosphate), and PaCO2. In this model, bicarbonate and pH are dependent variables that change in response to alterations in the independent variables. The strong ion difference is the net charge of fully dissociated cations minus fully dissociated anions, primarily (Na+ + K+) minus (Cl- + lactate). A decrease in strong ion difference produces metabolic acidosis, while an increase produces metabolic alkalosis. Albumin acts as a weak acid, so hypoalbuminemia generates a metabolic alkalosis that can mask an underlying acidosis when assessed by bicarbonate alone. The Stewart approach is particularly useful in critically ill patients with low albumin, where the anion gap may be falsely low and obscure a clinically significant lactic acidosis. ## Primary Metabolic Acidosis Metabolic acidosis is defined by a primary decrease in plasma bicarbonate concentration with a resulting fall in pH. The disorder arises from one of three mechanisms: addition of fixed acid, loss of bicarbonate from the gastrointestinal tract or kidneys, or failure of renal acid excretion. The anion gap separates these mechanisms at the bedside and directs the initial diagnostic plan. An elevated anion gap metabolic acidosis in dogs and cats is most commonly caused by lactic acidosis, diabetic ketoacidosis, uremic acidosis, or intoxication with ethylene glycol. Each cause has distinguishing features: lactate is measured directly, ketones are detected in blood or urine, uremia is identified by azotemia with a disproportionate acidosis, and ethylene glycol produces calcium oxalate crystalluria and a characteriztic osmolar gap. A normal anion gap metabolic acidosis is typically caused by gastrointestinal bicarbonate loss from diarrhea, renal tubular acidosis, or carbonic anhydrase inhibitor administration. The distinction matters because the therapeutic approach differs: elevated anion gap acidoses often resolve with treatment of the underlying cause, whereas normal anion gap acidoses may require alkali therapy to replace ongoing bicarbonate losses. ### Expected Compensation in Metabolic Acidosis The respiratory compensation for metabolic acidosis is hyperventilation, which lowers PaCO2 and partially restores pH. The expected PaCO2 can be estimated using Winter's formula, in which expected PaCO2 equals 1.5 times the measured bicarbonate plus 8, with a tolerance of plus or minus 2 mm Hg. This relationship holds in both dogs and cats. If the measured PaCO2 is higher than expected, a concurrent respiratory acidosis exists. If it is lower than expected, a concurrent respiratory alkalosis is present. The formula loses accuracy at extreme bicarbonate values, and clinical judgment should prevail when values fall outside the range where the relationship has been validated. ## Stepwise Interpretation of the Blood Gas Panel The interpretation sequence begins with sample validation. Confirm the reported pH, PCO₂, and HCO₃⁻ are internally consistent using the Henderson-Hasselbalch relationship, a mismatch indicates a pre-analytical error such as air exposure, delayed analysis, or an improperly heparinized sample. Next, assess pH to identify acidemia (pH below 7.35) or alkalemia (pH above 7.45). The direction of the primary disturbance is then determined by comparing PCO₂ and HCO₃⁻ against their reference intervals. A PCO₂ deviation explains a respiratory component, an HCO₃⁻ deviation explains a metabolic component. When both deviate in the same direction, one is compensatory. When they deviate in opposite directions, a mixed disorder exists. The expected compensation formulas provide the quantitative check. For metabolic acidosis, the expected PCO₂ in dogs is approximately 1.5 × HCO₃⁻ + 8, with a variance of ±2. In cats, the expected PCO₂ is approximately 1.5 × HCO₃⁻ + 8 as well, though feline compensation is often less complete. For metabolic alkalosis, the expected PCO₂ rises by 0.7 mm Hg per 1 mEq/L increase in HCO₃⁻. For respiratory disorders, the metabolic compensation is slower and follows different rules. Acute respiratory acidosis raises HCO₃⁻ by approximately 0.15 mEq/L per 1 mm Hg PCO₂ elevation, chronic respiratory acidosis raises it by 0.35 mEq/L per 1 mm Hg. The corresponding values for respiratory alkalosis are a decrease of 0.25 mEq/L acutely and 0.55 mEq/L chronically per 1 mm Hg PCO₂ reduction. These formulas are derived from human data and applied to small animals with acknowledged imprecision, but they remain clinically useful for distinguishing appropriate compensation from a superimposed primary disorder. ## The Anion Gap in Clinical Decision-Making The anion gap is calculated as (Na⁺ + K⁺), (Cl⁻ + HCO₃⁻). The reference interval is typically 12 to 24 mEq/L in dogs and 13 to 27 mEq/L in cats, though each laboratory should establish its own values. An elevated anion gap in metabolic acidosis indicates accumulation of unmeasured anions, most commonly lactate, ketoacids, uremic anions, or ethylene glycol metabolites. A normal anion gap metabolic acidosis indicates a primary bicarbonate loss, typically from the gastrointestinal tract or kidneys, or a failure of renal acid excretion with concurrent chloride retention. The delta gap, calculated as the change in anion gap divided by the change in bicarbonate, helps identify mixed metabolic disorders. A delta gap near 1 suggests a pure high anion gap acidosis. A delta gap below 1 indicates a concurrent normal anion gap acidosis. A delta gap above 1 suggests a concurrent metabolic alkalosis or a pre-existing elevated bicarbonate. This calculation is most useful when the clinician suspects multiple concurrent processes, such as a diabetic ketoacidosis patient with concurrent diarrhea or vomiting. The urine anion gap, calculated as (Na⁺ + K⁺), Cl⁻ in urine, can differentiate renal from extrarenal causes of normal anion gap metabolic acidosis. A negative urine anion gap indicates appropriate renal ammonium excretion and points to gastrointestinal bicarbonate loss. A positive urine anion gap suggests impaired renal acidification. This test requires careful urine collection and is less commonly performed in practice, but it can clarify ambiguous cases. ## Respiratory Acidosis Respiratory acidosis arises from alveolar hypoventilation. The hallmark is an elevated PCO₂ with a compensatory increase in HCO₃⁻. Acute causes include airway obstruction, severe pneumonia, pulmonary edema, neuromuscular disease, and central nervous system depression from drugs or trauma. Chronic causes include restrictive thoracic disease, chronic obstructive pulmonary disease, and severe obesity. The clinical distinction between acute and chronic respiratory acidosis relies on the magnitude of the HCO₃⁻ response and the patient history. A patient with chronic compensated respiratory acidosis may have a near-normal pH despite a markedly elevated PCO₂. Treatment targets the underlying ventilatory defect. Supplemental oxygen does not correct hypercapnia and may worsen hypoventilation in some patients with chronic CO₂ retention. Mechanical ventilation is indicated when hypoventilation is severe, when the patient is fatiguing, or when the underlying cause is not rapidly reversible. Monitoring includes serial blood gas analysis, pulse oximetry, and capnography where available. The response to therapy is assessed by the trend in PCO₂ and pH instead of by a single measurement. ## Respiratory Alkalosis Respiratory alkalosis results from alveolar hyperventilation. The hallmark is a decreased PCO₂ with a compensatory decrease in HCO₃⁻. Causes in dogs and cats include pain, anxiety, fever, sepsis, hepatic encephalopathy, and iatrogenic overventilation during anesthesia. High altitude exposure is a rare cause in small animals. The diagnosis is often made incidentally on a blood gas panel obtained for another reason, as the clinical signs of hyperventilation may be subtle. Treatment is directed at the underlying stimulus. Sedation or analgesia may reduce panting in anxious or painful patients. Correction of fever, sepsis, or hepatic disease addresses the primary process. There is no role for rebreathing techniques in small animal patients, as these are ineffective and distressing. The compensatory metabolic response develops over 24 to 48 hours, and the HCO₃⁻ will remain low until the respiratory stimulus resolves. ## Mixed Acid-Base Disorders Mixed disorders are common in critically ill patients and are identified when the measured compensation does not match the expected compensation. The most frequent combinations include respiratory acidosis with metabolic acidosis, as seen in cardiopulmonary arrest or severe sepsis with concurrent hypoventilation, and metabolic acidosis with respiratory alkalosis, as seen in sepsis or hepatic failure with concurrent lactate accumulation. Triple disorders, involving a respiratory disturbance and two metabolic disturbances, occur but are difficult to identify without a complete panel including albumin and lactate. The clinical approach to a suspected mixed disorder is to identify each component separately and treat each cause. The anion gap and delta gap calculations are essential here, as they can reveal a hidden metabolic alkalosis or a second metabolic acidosis. Serial blood gas measurements are more informative than a single panel, as the pattern of change over time clarifies whether compensation is appropriate or whether a second primary disorder is emerging. ## Monitoring and Documentation Serial blood gas analysis is the standard for monitoring acid-base status in hospitalized patients. The frequency depends on the severity of the disturbance and the stability of the patient. A patient with severe metabolic acidosis should be reassessed every 2 to 4 hours during initial stabilization, while a patient with chronic compensated respiratory acidosis may only need daily monitoring. Venous blood gas analysis is acceptable for assessing pH and HCO₃⁻ in most situations, but arterial sampling is required when assessing oxygenation or when the PCO₂ measurement must reflect alveolar ventilation precisely. Documentation should include the primary disturbance, the compensatory response, the calculated anion gap, and the specific etiologic diagnosis. The treatment plan should state the target pH and the expected timeline for correction. A table of common disturbances with their causes and compensation formulas is provided below. | Disorder | Primary Change | Compensatory Response | Common Causes | |---|---|---|---| | Metabolic acidosis, high anion gap | ↓ HCO₃⁻ | ↓ PCO₂ (1.5 × HCO₃⁻ + 8 ± 2) | Diabetic ketoacidosis, lactic acidosis, uremia, ethylene glycol | | Metabolic acidosis, normal anion gap | ↓ HCO₃⁻ | ↓ PCO₂ (1.5 × HCO₃⁻ + 8 ± 2) | Diarrhea, renal tubular acidosis, carbonic anhydrase inhibitors | | Metabolic alkalosis | ↑ HCO₃⁻ | ↑ PCO₂ (0.7 × ΔHCO₃⁻) | Vomiting, gastric dilatation-volvulus, diuretic therapy | | Acute respiratory acidosis | ↑ PCO₂ | ↑ HCO₃⁻ (0.15 × ΔPCO₂) | Airway obstruction, anesthesia, neuromuscular disease | | Chronic respiratory acidosis | ↑ PCO₂ | ↑ HCO₃⁻ (0.35 × ΔPCO₂) | Chronic lung disease, thoracic wall disease | | Acute respiratory alkalosis | ↓ PCO₂ | ↓ HCO₃⁻ (0.25 × ΔPCO₂) | Pain, anxiety, sepsis, hepatic encephalopathy | | Chronic respiratory alkalosis | ↓ PCO₂ | ↓ HCO₃⁻ (0.55 × ΔPCO₂) | Chronic hepatic disease, prolonged hyperventilation | The choice between arterial and venous sampling, the frequency of reassessment, and the decision to pursue advanced diagnostics such as thoracic imaging are guided by the patient's clinical trajectory instead of by the acid-base values alone. In patients with respiratory disease, thoracic computed tomography can characterize parenchymal and airway lesions more precisely than radiography, which may alter the treatment plan when the acid-base disturbance is secondary to an undiagnosed pulmonary process [Masseau and Reinero, thoracic CT interpretation for respiratory disease](https://pubmed.ncbi.nlm.nih.gov/31685132/). Similarly, in cats with suspected endocrine or renal contributions to metabolic acidosis, the diagnostic approach should incorporate species-specific considerations for calcium and parathyroid hormone testing, as feline assays and sample handling requirements differ from canine protocols [Parker, Gilor, and Chew, feline hyperparathyroidism diagnosis](https://pubmed.ncbi.nlm.nih.gov/25896242/). ## Recognized Complications and Early Detection The principal failure mode in acid-base management is misclassification of a mixed disorder as a simple one. This occurs most often when the clinician applies expected compensation ranges without first verifying that the measured response is physiologically plausible. Early detection depends on paired assessment of pH, PCO₂, and bicarbonate, with the anion gap calculated from the same sample. A widening anion gap that does not match the fall in bicarbonate suggests a concurrent metabolic alkalosis or a pre-existing compensated respiratory disorder. Serial blood gas panels, instead of single measurements, distinguish evolving compensation from a new primary process. A second recognized complication is overcorrection during therapy. Rapid bicarbonate administration in metabolic acidosis can produce overshoot alkalosis, hypokalemia, and paradoxical cerebrospinal fluid acidosis. Detection requires monitoring of electrolytes alongside blood gases, with particular attention to potassium and ionized calcium, since alkalosis drives both into cells. Similarly, aggressive ventilation in respiratory acidosis can precipitate acute alkalemia with cerebral vasoconstriction. Early detection relies on frequent reassessment after each ventilator or flow-by adjustment, with a target of gradual normalization instead of immediate correction. A third failure mode is the laboratory artefact. Venous samples with prolonged exposure to air lose CO₂ and raise pH, mimicking respiratory alkalosis. Samples analyzed after delayed processing show ongoing cellular metabolism, lowering pH and raising PCO₂. Detection requires correlation with the clinical picture and, when discordant, repeat sampling with immediate analysis. The [MSD Veterinary Manual](https://www.msdvetmanual.com/) provides species-specific reference intervals and sample handling guidance that should be consulted before interpreting borderline values. ## Common Errors and Corrective Action Less experienced clinicians frequently interpret the anion gap in isolation. A normal anion gap does not exclude metabolic acidosis, particularly in hyperchloremic acidosis from renal tubular disease or gastrointestinal bicarbonate loss. The corrective action is to calculate the gap and then compare it with the change in bicarbonate, a gap that is normal but with a low bicarbonate should prompt evaluation of chloride. Conversely, a high anion gap with a normal bicarbonate suggests a mixed disorder, not a simple one. A second common error is treating the number instead of the patient. A mildly abnormal pH with adequate compensation may not require intervention, while a normal pH in the setting of a wide anion gap and abnormal PCO₂ indicates a mixed disorder that demands investigation. The corrective action is to apply the stepwise interpretation framework consistently, including assessment of the primary disorder, the compensatory response, and the anion gap, before any therapeutic decision. A third error is failure to account for species differences. Cats with respiratory disease may show substantial pulmonary hemosiderosis on bronchoalveolar lavage, and this finding should prompt evaluation for underlying cardiovascular or inflammatory disease instead of being attributed solely to the primary respiratory process, as documented in a retrospective study of bronchoalveolar lavage samples in dogs and cats [bronchoalveolar lavage hemosiderosis in dogs and cats with respiratory disease](https://pubmed.ncbi.nlm.nih.gov/30657606/). The corrective action is to integrate acid-base data with thoracic imaging findings, using the systematic interpretation roadmap described for computed tomography in dogs and cats [thoracic computed tomographic interpretation for clinicians](https://pubmed.ncbi.nlm.nih.gov/31685132/). ## Troubleshooting Table | Observation | Likely Cause | Discriminating Check | |---|---|---| | High pH, low PCO₂, normal bicarbonate | Acute respiratory alkalosis | Confirm with clinical signs, repeat sample if artefact suspected | | Low pH, high PCO₂, high bicarbonate | Chronic respiratory acidosis with metabolic compensation | Compare bicarbonate against expected range for duration | | Normal pH, high anion gap, low bicarbonate | Mixed metabolic acidosis and alkalosis | Calculate delta gap, assess chloride and albumin | | High anion gap, normal bicarbonate | Mixed disorder or pre-existing alkalosis | Review serial blood gases and electrolyte trends | | Low bicarbonate, normal anion gap | Hyperchloremic metabolic acidosis | Measure chloride, evaluate renal and gastrointestinal causes | | Rising pH after bicarbonate therapy | Overshoot alkalosis | Check potassium and ionized calcium, reduce or stop therapy | ## Evidence Limitations and Expert Disagreement The evidence base for acid-base interpretation in dogs and cats rests largely on extrapolation from human medicine and on small retrospective studies. Expert opinion still differs on the preferred interpretive framework. Traditional Henderson-Hasselbalch analysis remains the clinical standard, while the Stewart approach offers additional insight into strong ion and albumin effects but has not been shown to improve outcomes in veterinary patients. Clinicians should choose one framework and apply it consistently instead of switching between methods. There is also genuine uncertainty regarding optimal targets for compensation in chronic respiratory disorders. Published expected ranges derive from limited patient cohorts, and individual variation is substantial. The [ACVIM consensus statements](https://www.acvim.org/Animal-Owners/Animal-Education/Consensus-Statements) provide expert guidance where controlled data are lacking, and practitioners should consult these when managing complex or refractory cases. ## Referral and Escalation Criteria Referral or specialist consultation is warranted when a mixed acid-base disorder is suspected but the underlying cause remains unidentified after initial investigation, when serial blood gas measurements show progressive deterioration despite treatment, or when the patient requires mechanical ventilation. Laboratory involvement is appropriate when electrolyte measurements are inconsistent with the clinical picture, when ionized calcium or magnesium measurement is needed, or when the laboratory's analyzer produces results that conflict with clinical expectations. Regulatory reporting may be required when acid-base disturbances arise from suspected toxic exposures, including ethylene glycol, or from notifiable diseases. The [WOAH terrestrial animal health standards](https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/) define reporting obligations for listed diseases, and the [AVMA practice resources](https://www.avma.org/resources-tools) provide guidance on professional responsibilities in such cases. Clinicians should confirm local requirements, as these vary by region and species. ## Frequently Asked Questions ### How Do I Interpret Acid-Base Data When Only Venous Blood Gas Is Available? Venous blood gas analysis is acceptable for assessing metabolic acid-base status in most clinical scenarios. Venous pH is typically 0.02 to 0.05 units lower than arterial pH, and venous PCO2 is 4 to 6 mmHg higher. The bicarbonate concentration and base excess are comparable between arterial and venous samples. Venous samples cannot reliably assess respiratory function, so respiratory acid-base disorders require arterial sampling or careful clinical correlation. When interpreting venous PCO2, account for the expected veno-arterial gradient before concluding that respiratory acidosis exists. Serial venous samples are useful for monitoring response to therapy in metabolic disorders. The [MSD Veterinary Manual](https://www.msdvetmanual.com/) provides species-specific reference intervals for venous blood gas parameters. ### What Should I Do When Blood Gas Analysis Is Unavailable? When a blood gas analyzer is unavailable, serum biochemistry provides a workable substitute. Measure total CO2 (TCO2) on the chemistry panel, which approximates bicarbonate concentration. A low TCO2 with a high anion gap supports metabolic acidosis. A high TCO2 with hypochloremia suggests metabolic alkalosis. Calculate the anion gap from measured electrolytes: (Na+ + K+) minus (Cl- + TCO2). This approach cannot detect respiratory disorders or mixed acid-base disturbances reliably. For patients with suspected respiratory disease, pulse oximetry and clinical assessment of ventilation must substitute for blood gas analysis. Document the limitation in the medical record and note that respiratory compensation cannot be quantified. The [ACVIM consensus statements](https://www.acvim.org/Animal-Owners/Animal-Education/Consensus-Statements) offer guidance on diagnostic approaches when advanced testing is limited. ### How Does the Approach Differ Between Dogs and Cats? Cats have a narrower range of normal pH and bicarbonate values than dogs, and they compensate more slowly for metabolic acidosis. Feline renal ammonium excretion is more limited, so cats with renal disease develop acidosis at earlier stages of dysfunction. Cats also have a higher incidence of diseases that produce mixed acid-base disorders, such as chronic kidney disease with concurrent gastrointestinal losses. The anion gap reference interval differs slightly between species, so use species-specific values. Cats with diabetic ketoacidosis frequently have a lower anion gap than dogs with the same condition because of concurrent hyperchloremia. When interpreting feline acid-base data, consider that stress hyperventilation during sampling can produce a transient respiratory alkalosis that complicates interpretation. ### What Are the Most Common Causes of a Normal Anion Gap Metabolic Acidosis in Practice? Gastrointestinal bicarbonate loss from diarrhea is the most frequent cause in both dogs and cats. Renal tubular acidosis, though less common, should be considered when diarrhea is absent. Carbonic anhydrase inhibitor therapy, such as acetazolamide, produces a normal anion gap acidosis. Early chronic kidney disease can present with a normal anion gap before uremic anions accumulate. Dilutional acidosis from rapid administration of large volumes of 0.9% sodium chloride is an iatrogenic cause. Urine pH and urine ammonium measurement help distinguish renal from gastrointestinal causes. A urine pH above 6.5 in the face of systemic acidosis suggests renal bicarbonate wasting or impaired acidification. The [MSD Veterinary Manual](https://www.msdvetmanual.com/) lists differential diagnoses for normal anion gap acidosis by species. ### How Should I Document Acid-Base Interpretation in the Medical Record? Record the primary acid-base disorder, the calculated compensation status, and the anion gap with the reference interval used. State whether the compensation is appropriate, inadequate, or excessive, and list the differential diagnoses considered for each component. Document the specific treatment administered and the planned monitoring interval. Include the time of sampling relative to treatment initiation, because therapy changes the interpretation of subsequent samples. Note any factors that limit interpretation, such as hemolyzed samples or venous sampling for respiratory assessment. Serial records should show trends in pH, PCO2, bicarbonate, and anion gap instead of isolated values. This documentation supports clinical decision-making and provides a defensible record if the case is reviewed. ### How Do I Explain Acid-Base Abnormalities to an Owner or Referring Veterinarian? Use analogies that preserve the physiology without oversimplifying. Describe acidosis as the blood becoming too acidic, which impairs enzyme function and organ performance. Explain that the kidneys and lungs normally correct this, and that treatment supports these systems while addressing the underlying cause. For referring veterinarians, provide the primary diagnosis, the specific acid-base parameters, and the treatment plan in writing. State clearly which parameters require monitoring and at what interval. If referral is indicated, provide the blood gas data and the treatments already administered. The [AVMA practice resources](https://www.avma.org/resources-tools) offer guidance on professional communication and referral documentation standards. ## Related Clinical & Scientific Guides * [Feline Hepatic Lipidosis: Nutritional and Medical Management](/knowledge/veterinary-medicine/clinical-internal-medicine/feline-hepatic-lipidosis-nutritional-medical-management) * [Canine Respiratory Infection: Diagnostic Approach and Treatment](/knowledge/veterinary-medicine/clinical-internal-medicine/canine-respiratory-infection-diagnostic-approach-treatment) * [Canine Respiratory Virus: Diagnostic and Management Considerations](/knowledge/veterinary-medicine/clinical-internal-medicine/canine-respiratory-virus-diagnostic-management-considerations) ## References and Further Reading - [Thoracic computed tomographic interpretation for clinicians to aid in the diagnosis of dogs and cats with respiratory disease.](https://pubmed.ncbi.nlm.nih.gov/31685132/). 2019. - [Feline hyperparathyroidism: pathophysiology, diagnosis and treatment of primary and secondary disease.](https://pubmed.ncbi.nlm.nih.gov/25896242/). 2015. - [A retrospective study of 286 cases of neurological disorders of the cat.](https://pubmed.ncbi.nlm.nih.gov/15276850/). 2004. - [Feline vestibular disorders. Part I: anatomy and clinical signs.](https://pubmed.ncbi.nlm.nih.gov/11919020/). 1999. - [A surveillance system for diseases of companion animals in the Veneto region (Italy).](https://pubmed.ncbi.nlm.nih.gov/30160683/). 2017. - [Bronchoalveolar lavage hemosiderosis in dogs and cats with respiratory disease.](https://pubmed.ncbi.nlm.nih.gov/30657606/). 2019. - [ACVIM Consensus Statements](https://www.acvim.org/Animal-Owners/Animal-Education/Consensus-Statements). Journal of Veterinary Internal Medicine. - [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. ## Related Articles - [Canine Vomiting: Home Management and When to Seek Veterinary Care](/knowledge/veterinary-medicine/clinical-internal-medicine/canine-vomiting-home-management-when-seek-veterinary-care) - [Canine Vomiting: Therapeutic Management and Antiemetic Selection](/knowledge/veterinary-medicine/clinical-internal-medicine/canine-vomiting-therapeutic-management-antiemetic-selection) - [Canine and Feline Protein-Losing Enteropathy: Diagnostic and Management Approach](/knowledge/veterinary-medicine/clinical-internal-medicine/canine-feline-protein-losing-enteropathy-diagnostic-management) - [Cardiac Biomarker Interpretation in Canine and Feline Practice](/knowledge/veterinary-medicine/clinical-internal-medicine/cardiac-biomarker-interpretation-canine-feline) - [Canine and Feline Acute Vomiting: Triage and Diagnostic Approach](/knowledge/veterinary-medicine/clinical-internal-medicine/canine-feline-acute-vomiting-triage-diagnostic) > 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.