Veterinary Clinical Pathology: Interpreting the Numbers
Core concepts
Clinical pathology questions reward disciplined interpretation of laboratory data. Candidates must understand hematology (erythrogram: anemia classification by regeneration and morphology; leukogram: stress vs. inflammatory vs. physiologic patterns; platelet evaluation), hemostasis (primary vs. secondary hemostasis, appropriate tests), serum biochemistry (hepatic, renal, pancreatic, and muscle markers; electrolytes and acid-base), urinalysis (concentration, sediment, casts, crystals), and cytology (inflammation vs. neoplasia, cell criteria).
Traps in questions
- Regenerative vs. nonregenerative anemia: reticulocyte count is the discriminator; species differences matter-cats require stronger reticulocyte responses and horses have limited marrow reserve.
- Stress leukogram: mature neutrophilia with lymphopenia, eosinopenia, and monocytosis-do not mislabel as inflammation. Inflammation often shows left shift or toxic change.
- Azotemia vs. renal failure: prerenal azotemia concentrates urine (high USG); renal azotemia is isosthenuric. A single value rarely diagnoses renal disease.
- Enzyme leakage vs. induction: ALT leakage reflects hepatocellular injury; ALP may reflect cholestasis or induction (e.g., corticosteroids, anticonvulsants).
- Cytology limits: cannot assess tissue architecture; "cannot rule out" is often the honest answer.
- Sample artifacts: hemolysis, lipemia, and delayed processing alter results; recognize them before interpreting.
Worked example / rule of thumb
A dog with vomiting: BUN and creatinine are elevated. Check urine specific gravity. High USG (>1.030) with concentrated urine supports prerenal azotemia (dehydration); isosthenuria (~1.008–1.012) supports renal azotemia. Rule of thumb: interpret each abnormality against the patient (signalment, hydration, medications) and against the other data-never in isolation.