Laboratory Procedures
Domain focus
Laboratory Procedures tests sample collection, handling, quality control, and interpretation of hematology, chemistry, urinalysis, cytology, and coagulation tests. The exam emphasizes preanalytical errors because they change results before the analyzer runs.
Core rules and concepts
- Use EDTA for CBCs, serum separator or lithium heparin for most chemistry panels, and sodium citrate for coagulation testing. Fill citrate tubes to the marked line because the 9:1 blood-to-anticoagulant ratio is critical.
- Process samples promptly; label tubes at the patient's side; avoid hemolysis by not forcing blood through a small needle.
- Run quality control materials before patient samples and record results. Reject clotted, underfilled, or mislabeled samples.
- Correct the leukocyte count for nucleated red blood cells: corrected WBC = measured WBC × 100 ÷ (100 + nRBC per 100 WBC).
- Urinalysis includes color, clarity, specific gravity, dipstick chemistry, and sediment examination. Refrigerate if delay is unavoidable, but allow the sample to reach room temperature before analysis.
Worked example
An automated analyzer reports a WBC count of 20,000/µL, and the differential shows 20 nucleated RBCs per 100 WBCs. Corrected WBC = 20,000 × 100 ÷ 120 = 16,667/µL. The technician reports the corrected value and notes the nRBCs, because the uncorrected count overestimates leukocytes.
Traps that cost marks
Using a serum tube for a CBC; underfilling a citrate tube; interpreting lipemia as true hyperglycemia; and reporting a count without checking the blood film.