Clinical Chemistry, Urinalysis, Parasitology, and Microbiology Procedures
Key Takeaways
- ALT is a liver-leakage enzyme specific for hepatocellular injury in dogs and cats; ALP is an inducible cholestatic enzyme that also rises with bone growth and corticosteroid exposure in dogs.
- Creatinine is more specific for glomerular filtration rate than BUN because BUN is affected by protein intake, GI hemorrhage, and dehydration; both rise only after approximately 75% of nephrons are lost.
- Lipemia and hemolysis falsely alter chemistry results — hemolysis elevates potassium (intracellular release) and falsely lowers ALT and glucose; lipemia scatters light and interferes with photometric assays.
- Fecal centrifugal flotation with zinc sulfate or sugar solution (SG 1.18–1.20) detects most nematode and protozoal eggs, but tapeworm eggs are heavy and frequently missed — use a fecal antigen SNAP test or visualize proglottids.
- First-morning urine is the preferred sample for urinalysis because it is the most concentrated, maximizing the chance of detecting abnormalities; cystocentesis is the only collection method that avoids lower-tract contamination.
Quick Answer: The laboratory procedures domain tests your ability to interpret clinical chemistry panels, perform a complete urinalysis, select the right fecal parasite detection method, and process microbiology specimens correctly. The most frequently tested traps involve preanalytical errors (lipemia/hemolysis), knowing which enzyme is organ-specific versus inducible, and recognizing that centrifugal flotation is more sensitive than passive flotation but still misses tapeworms.
Clinical Chemistry Panels
The serum biochemistry panel organizes analytes by organ system. Each marker has a tissue origin, a half-life, and a mechanism of elevation that the VTNE tests directly.
| Marker | Tissue | Rises With | VTNE Trap |
|---|---|---|---|
| ALT | Hepatocellular (liver) | Hepatocellular injury — leaking from damaged hepatocytes | Liver-specific in dogs/cats; muscle ALT exists but is minor |
| ALP | Liver, bone, intestine, placenta | Cholestasis, bone growth (young animals), corticosteroid induction (dogs), endocrine disease | Corticosteroid-induced ALP isoenzyme in dogs can falsely suggest liver disease |
| GGT | Liver (membrane-bound) | Cholestasis — parallels ALP but more liver-specific | More sensitive than ALP for cholestasis in cats |
| Total bilirubin | RBC breakdown, liver conjugation | Hemolysis (prehepatic), hepatocellular dysfunction, biliary obstruction | Split into direct (conjugated) and indirect (unconjugated) to localize cause |
| BUN | Liver (synthesis) | Renal failure, dehydration, GI hemorrhage, high-protein meal | Affected by non-renal factors; not a pure GFR marker |
| Creatinine | Muscle breakdown | Renal failure — freely filtered, not reabsorbed | More GFR-specific than BUN; influenced by muscle mass |
| Phosphorus | Bone, diet | Renal failure (decreased excretion), growing animals | Inversely related to calcium; hyperphosphatemia drives hypocalcemia in CKD |
| Glucose | Carbohydrate metabolism | Diabetes mellitus, stress hyperglycemia (cats), IV dextrose | Stress hyperglycemia in cats can exceed 300 mg/dL without diabetes |
| Lipase | Pancreas | Pancreatitis (more specific than amylase) | Spec cPL or fPL immunoassays are the gold standard for pancreatitis |
| Total protein / Albumin | Liver (albumin synthesis) | Dehydration (high), liver disease/nephrotic syndrome/protein-losing enteropathy (low) | Albumin has long half-life (~8 days in dogs); acute liver injury may not drop albumin |
Electrolytes complete the panel: sodium (hypernatremia in water deprivation, hyponatremia in Addison's), potassium (hyperkalemia in urethral obstruction, hypoadrenocorticism, AKI; hypokalemia in vomiting/diarrhea and feline CKD), and chloride (parallel to sodium; corrected for anion gap). Bicarbonate or total CO2 reflects acid-base status.
Preanalytical errors are heavily tested. Hemolysis releases intracellular contents (potassium, AST, LDH) from RBCs, falsely elevating potassium and lowering ALT and glucose (red cells consume glucose in vitro). Lipemia scatters light in photometric assays, falsely elevating or lowering analytes depending on the assay. Icterus (hemolysis/bilirubin) interferes with colorimetric reactions. If a sample is grossly hemolyzed or lipemic, note it on the submission and consider recollection.
Urinalysis
A complete urinalysis has three components: physical characteristics, chemical (dipstick), and sediment microscopic examination. First-morning urine is preferred because it is the most concentrated, maximizing detection of abnormalities.
Collection methods determine what is normal to find:
- Cystocentesis — percutaneous bladder puncture; gold standard for culture (sterile) and avoids lower-tract contamination; can introduce iatrogenic hematuria and is contraindicated with coagulopathy or bladder neoplasia.
- Urethral catheterization — introduces bacteria; not ideal for culture without quantitative interpretation.
- Free catch / voided — easiest but contaminated with lower-tract and environmental flora; not acceptable for sterile culture.
Specific gravity is measured by refractometer (more accurate than dipstick, which only estimates). Isosthenuria (1.008–1.012) indicates loss of concentrating ability — renal failure, diabetes insipidus. Hyposthenuria (<1.008) indicates the kidney is actively diluting urine. Hypersthenuria (>1.030 in dogs, >1.035 in cats) indicates active concentration — dehydration or renal-conserving states.
Dipstick pads include pH, protein, glucose, ketones, blood/hemoglobin, bilirubin, urobilinogen, nitrite, and leukocyte esterase. The protein pad is most reliable at alkaline pH; significant proteinuria should be confirmed with a urine protein-to-creatinine ratio (UPC). The leukocyte esterase and nitrite pads are unreliable in veterinary species.
Sediment examination uses low-speed centrifugation (around 1,500 rpm for 5 minutes); the supernatant is decanted and the pellet resuspended for microscopy:
- Crystals — struvite (triple phosphate, magnesium ammonium phosphate, associated with alkaline urine and UTI), calcium oxalate dihydrate and monohydrate (acidic urine, ethylene glycol toxicity for monohydrate), ammonium biurate (liver disease/portosystemic shunt, especially in Dalmatians and cats).
- Casts — formed in distal tubules; hyaline casts (benign, protein), granular casts (tubular degeneration), cellular casts (RBC, WBC, epithelial — active renal injury), waxy/broad casts (chronic renal disease).
- Cells — RBCs (>5/hpf is abnormal), WBCs (>5/hpf suggests inflammation/infection), epithelial cells (squamous are contamination; transitional cells are bladder/urethral).
- Organisms — bacteria visible at 40× suggest significant bacteriuria; yeast (Candida) is rare.
Parasitology
Fecal flotation exploits the density difference between parasite eggs/cysts and the flotation solution. Sugar solution (Sheather's, SG 1.18–1.20) or zinc sulfate (SG 1.18) are standard. Centrifugal flotation is more sensitive than passive flotation because the centrifugal force accelerates egg rise and increases recovery. Zinc sulfate preserves Giardia cysts better than sugar (sugar distorts them).
McMaster counting is a quantitative flotation method used for fecal egg counts — eggs per gram (EPG). It is primarily used in large-animal practice to determine deworming necessity and anthelmintic resistance (Fecal Egg Count Reduction Test).
Fecal antigen tests (SNAP Giardia, SNAP parvovirus) detect pathogen antigens directly. Tapeworm eggs are heavy, have high specific gravity, and frequently fail to float — a negative flotation does not rule out tapeworms. Visualize proglottids on the feces or perianal region, or use a fecal antigen test.
Direct fecal smear examines fresh feces for motile trophozoites (Giardia, Tritrichomonas foetus in cats) and is useful for heavy infections but insensitive for routine screening.
Microbiology
The Gram stain classifies bacteria: gram-positive (thick peptidoglycan, stains purple — Staphylococcus, Streptococcus, Clostridium, Corynebacterium) versus gram-negative (thin peptidoglycan, stains pink/red — E. coli, Salmonella, Pseudomonas, Klebsiella). This guides initial antibiotic selection while culture results are pending.
Culture and sensitivity identifies the organism and its antibiotic susceptibilities. Disk diffusion (Kirby-Bauer) measures zones of inhibition around antibiotic disks; broth dilution determines the minimum inhibitory concentration (MIC). The MIC is the lowest antibiotic concentration that inhibits visible growth — more quantitative than disk diffusion.
Transport media preserve organism viability without overgrowth. Cary-Blair transport medium is used for enteric pathogens (Salmonella, Campylobacter) in stool. Amies or Stuart medium is used for swabs. Fastidious organisms require enriched media: blood agar for most bacteria, MacConkey for gram-negative enterics, chocolate agar for fastidious organisms (Haemophilus, Pasteurella), and anaerobic culture requires special handling (transport in anaerobic tubes, plate on Brucella agar).
A serum chemistry panel from a 10-year-old Cairn Terrier shows marked hemolysis. Which analyte is most likely falsely elevated, and what is the mechanism?
A 4-year-old cat presents with chronic diarrhea. Fecal flotation with sugar solution is negative. The veterinarian suspects Giardia. Which approach is most appropriate?
On a urinalysis from a 6-month-old Dalmatian, you observe yellow-brown spiculated crystals in acidic urine. What is the likely crystal type and associated condition?
Which statement correctly describes the difference between calibration and a control in laboratory quality assurance?