Specimen Collection and Handling (Pre-Analytical)
The Specimen Collection and Handling domain covers the pre-analytical phase, where the majority of laboratory errors originate. The exam tests order of draw, patient identification and labelling, collection technique, transport requirements and defensible rejection criteria - usually as "what should you do" scenarios.
Order of draw (CLSI GP41)
- Blood culture bottles
- Sodium citrate (light blue)
- Serum tube with or without clot activator/gel (red, gold, SST)
- Heparin (green)
- EDTA (lavender)
- Fluoride/oxalate (grey)
The sequence prevents additive carryover between tubes. EDTA carried into a chemistry tube falsely raises potassium and lowers calcium and magnesium. When a butterfly set is used and only a citrate tube is drawn, draw a discard tube first to fill the tubing dead space.
Collection essentials
- Identify with two identifiers - the patient states name and date of birth; label at the bedside, in the patient's presence, immediately after collection. Never label before drawing.
- Tourniquet on for no more than one minute, released before the needle is withdrawn.
- Cleanse with 70% isopropyl alcohol or chlorhexidine and let it dry; do not repalpate the site.
- Fill tubes to the marked volume; the citrate tube must reach the fill line to keep the 9:1 blood-to-anticoagulant ratio. Invert gently the number of times stated by the manufacturer - never shake.
- Capillary: wipe away the first drop; in infants under one year puncture the medial or lateral plantar heel, never the arch or fingers.
Handling and transport
Haemolysis arises from small-bore needles, excessive vacuum, shaking, IV-line draws and delayed separation. Protect light-sensitive analytes (bilirubin, B12, folate, porphyrins) with foil. Blood cultures are never refrigerated; keep them near 35–37 °C.
Rejection criteria
Unlabeled or mislabeled specimen, wrong tube or anticoagulant, clotted EDTA sample, short-draw citrate tube, unacceptable haemolysis, expired tube, leaked container, or a delay beyond the analyte's stability.
Worked example
A light blue tube arrives filled to roughly half the label line. Reasoning: the tube holds a fixed amount of sodium citrate calculated for nine parts blood to one part anticoagulant. Underfilling raises the anticoagulant-to-blood ratio, which falsely prolongs PT and aPTT and can lead to a patient being over-anticoagulated. The specimen must be rejected and recollected; it cannot be corrected or "topped up."
Traps that cost marks
- Labelling tubes before entering the room.
- Drawing EDTA before a chemistry tube.
- Leaving the tourniquet on for several minutes (haemoconcentration).
- Refrigerating blood cultures or removing the first capillary drop.