Phlebotomy: Order of Draw, Site Selection, and Specimen Integrity
Core concepts
Candidates must know the CLSI order of draw for venous collections: blood culture bottles first (to protect sterility), then light-blue sodium citrate tubes, serum tubes (red/gold, with or without clot activator or gel), heparin tubes (green), EDTA tubes (lavender/pink), and finally glycolytic inhibitor tubes (gray). Additive carryover is the reason for the sequence: citrate and heparin can contaminate downstream tubes and skew coagulation or chemistry results, and EDTA carryover introduces chelated calcium and potassium into serum or heparin samples. Understand additive mechanisms: citrate binds calcium reversibly; EDTA chelates calcium more strongly; heparin potentiates antithrombin; oxalate/fluoride inhibit glycolysis.
Site selection and technique
Know the antecubital anatomy (median cubital preferred, then cephalic, then basilic - the basilic lies near the brachial artery and median nerve). Tourniquet time should stay under one minute; prolonged stasis hemoconcentrates and falsely elevates potassium, protein, and cell counts. Know the sequence for capillary collection (wipe away first drop), heel vs finger puncture limits in infants, and that hemolysis falsely elevates potassium, LDH, and AST while lowering glucose.
Traps
Questions love the "short draw" citrate tube - underfilled tubes have excess anticoagulant relative to blood and prolong clotting times; the tube must be discarded and recollected. Another trap: drawing above an IV line or from a hematoma, and confusing the lavender (EDTA) tube's role in hematology with the light-blue tube's role in coagulation.
Worked example
A patient needs a PT/INR and a CBC. Draw order: light blue (citrate) before lavender (EDTA). If the citrate tube fills only halfway, reject it - the result would be falsely prolonged, not shortened.