Specimen Collection and Handling
Specimen collection and handling is the preanalytical domain, and it is where the majority of laboratory errors begin. The exam tests whether you can choose the correct tube and additive, draw tubes in the correct sequence, keep the specimen within stability limits, and reject what cannot be trusted with a defensible rationale.
Order of draw (CLSI GP41)
- Blood culture bottles first, always, to protect sterility.
- Light blue (3.2% sodium citrate) next: coagulation testing is the most additive-sensitive.
- Serum tubes (red, gold/SST) - allow 30 minutes for clotting.
- Green (lithium heparin) for stat chemistry and plasma assays.
- Lavender (K2/K3 EDTA) for hematology.
- Gray (sodium fluoride/potassium oxalate) for glucose and lactate. The sequence stops additive carryover on the needle: EDTA carried into a chemistry tube falsely raises potassium and lowers calcium; heparin carried into a citrate tube falsely prolongs the APTT.
Matching additive to test
Citrate binds calcium, so the tube must be filled to at least 90% of nominal volume to preserve the 9:1 blood-to-anticoagulant ratio. Heparin potentiates antithrombin and is unacceptable for coagulation testing. EDTA chelates divalent cations, preserving cells for CBC and reticulocyte counts, but falsely lowers calcium and can inhibit alkaline phosphatase. Serum is preferred when the analyte is unstable in plasma or the assay is calibrated for serum.
Transport and rejection
Separate serum or plasma from cells within 2 hours when the analyte is intracellular (potassium, LDH, AST) or consumed by cells (glucose). Most separated chemistry specimens hold at 2-8 °C, but coagulation specimens stay at room temperature and are tested within 4 hours because cold activates platelets and degrades factors. Reject unlabeled or mislabeled tubes, hemolyzed specimens for potassium or coagulation, short-draw citrate tubes, and anything beyond stated stability.
Worked example
A lavender EDTA tube is drawn before a gold SST for a chemistry panel. Potassium is 7.1 mmol/L, ionized calcium is low, and the patient's potassium was 4.0 yesterday. Spuriously high potassium with low calcium in a patient with normal renal function points to EDTA contamination. Reject and redraw in the correct order rather than calling a critical value.
Traps that cost marks
- Reporting a critical potassium from a hemolyzed or EDTA-contaminated tube.
- Forgetting the discard tube when a butterfly set is used before a light blue draw.
- Tourniquet left on longer than one minute: hemoconcentration raises protein, potassium, and RBC count.
- Assuming refrigeration preserves everything; cold agglutinins and labile coagulation factors behave differently.