Laboratory Procedures
Laboratory procedures questions cover sample choice, handling, and basic interpretation. Many errors happen before the sample reaches the analyzer, so the exam tests tube colors, order of draw, and artifact recognition.
Core rules
- EDTA (lavender) for CBC; heparin (green) for some chemistry and blood gas; serum separator (red/gold) for chemistry; citrate (light blue) for coagulation; oxalate/fluoride (gray) for glucose.
- Order of draw for venous samples: blood culture, citrate, serum, heparin, EDTA, fluoride/oxalate.
- Mix tubes gently by inversion; do not shake, which hemolyzes red cells.
- PCV and total protein are rapid cage-side tests; a blood smear assesses cell morphology and platelet estimates.
- Urinalysis includes specific gravity, dipstick, and sediment exam; refrigerate if not run promptly, but allow to room temperature before analysis.
Worked example
A CBC reports WBC 12,000/µL with 70% neutrophils. Absolute neutrophils = 12,000 × 0.70 = 8,400/µL. Reasoning: percentages describe distribution, not absolute numbers. Always convert to absolute counts before calling a value high or low, and compare with the laboratory's reference interval for the species.
Traps
- Sending EDTA blood for chemistry; EDTA chelates calcium and other analytes.
- Reading hemolysis or lipemia as true disease without noting the artifact.
- Delaying smear preparation, which distorts cells.
- Using a urine dipstick result alone without sediment or specific gravity.