Patient Preparation and Specimen Collection
Preanalytic error accounts for the majority of laboratory errors, so questions frequently test specimen requirements. Know fasting requirements (typically 8–12 hours for glucose and lipids), timing of draws relative to drug dosing (trough levels before next dose), and diurnal variation (cortisol, iron). Understand posture and tourniquet effects: prolonged tourniquet application concentrates proteins and potassium; hemolysis falsely elevates potassium, LDH, and AST while lowering glucose and potassium in some systems.
Collection tube order (order of draw) matters to avoid carryover: blood culture, then coagulation (light blue citrate), serum (red/gold), heparin (green), EDTA (lavender), and fluoride oxalate (gray). Citrate tubes must be filled to the mark; underfilling raises the citrate-to-blood ratio and falsely prolongs clotting times. EDTA contamination from a lavender tube drawn before a chemistry tube falsely lowers calcium and raises potassium.
Special handling: ammonia and blood gases on ice, light protection for bilirubin, prechilled tubes for catecholamines, and avoidance of hemolysis for potassium and lactate dehydrogenase. Timing of cultures, viral load, and therapeutic drug monitoring depends on pharmacokinetics.
Worked example: a potassium of 6.8 mmol/L with a normal sodium and no ECG changes in a well patient should prompt review of the draw; a hemolyzed sample or EDTA carryover is likely. Repeat with a fresh, properly ordered draw before treating.
Trap: assuming a single abnormal result reflects disease-preanalytic variables frequently explain isolated abnormalities.