Specimen Collection and Processing
Collection variables that change results
Specimen collection is an analytical step, not a clerical one. Candidates must know the CLSI order of draw for venous blood: blood culture bottles, sodium citrate (light blue), serum tube with or without clot activator/gel (red, gold, tiger), heparin (green), EDTA (lavender), glycolytic inhibitor (gray), and other additives. Additive carryover and order errors can produce EDTA contamination, which falsely lowers calcium and raises potassium. Tourniquet time should be under 1 minute; prolonged stasis hemoconcentrates proteins and cells and raises potassium. Needle gauge and vacuum are matched: small gauges increase hemolysis, especially with fragile veins. Citrate tubes must be filled to the marked volume because the 9:1 blood-to-anticoagulant ratio preserves coagulation testing; short draws give falsely prolonged PT/aPTT. Never pour blood from one tube to another. Capillary collections have their own order and avoid squeezing tissue, which dilutes with interstitial fluid and hemolyzes. Urine, CSF, and microbiology specimens have transport and temperature limits; delays reduce viability and change cell counts. Rejection criteria include unlabeled or mislabeled tubes, wrong additive, gross hemolysis, clotted anticoagulated specimens, and expired collection devices.
Traps and a worked rule
A common trap is an elevated potassium in a lavender-top tube with a normal serum potassium: suspect EDTA contamination or hemolysis rather than true hyperkalemia. Another is a short-draw citrate tube with a prolonged aPTT. Worked rule: for a 3.2% sodium citrate tube, the target is 9 parts whole blood to 1 part citrate; if the tube is underfilled, reject and recollect. For patients with hematocrit above 55%, the citrate volume must be reduced per validated laboratory protocol. Rule of thumb: if the collection cannot be repeated and the result will change care, document the limitation and escalate; do not silently report a compromised specimen.