Phlebotomy and Specimen Collection
Order of draw
The order exists to prevent additive carryover. Standard sequence: blood culture bottles (or sterile yellow SPS tube), then light blue sodium citrate, then serum tubes (red, gold/SST), then green heparin, then lavender EDTA, then gray sodium fluoride/oxalate. Capillary order differs - EDTA first among additive tubes, then other additives, then non-additive tubes last.
Technique that protects results
Verify identity with two identifiers and label at the bedside. Apply the tourniquet for less than one minute; prolonged tourniquet falsely raises potassium, protein, and calcium and lowers pH. Choose a needle large enough to avoid hemolysis, never draw from an arm with a running IV line on the same side, and fill citrate tubes to the mark because the 9:1 blood-to-anticoagulant ratio determines the clotting times. Allow alcohol to dry before puncture.
Complications and rejection criteria
Hematoma, syncope, petechiae, and nerve injury are common; ask about anticoagulants, mastectomy, fistulas, and prior syncope before choosing a site. Reject unlabeled tubes, grossly hemolyzed specimens, clotted anticoagulated tubes, and short-draw citrate tubes. Note collection time for drugs with narrow therapeutic windows and timed assays.
Traps
The favorite exam scenario is EDTA contamination of a coagulation tube: potassium rises, calcium falls, and PT/aPTT are prolonged - recollect in the correct order. Another is hemolysis from a 23-gauge butterfly with a syringe; the potassium is spuriously high and the sample is unsuitable.
Worked example: a light blue tube is half full and the aPTT is prolonged with a normal PT. Do not report and do not repeat the test on the same tube; recollect a full-draw citrate specimen and repeat.