Phlebotomy: Order of Draw, Technique, and Preanalytic Error
The phlebotomy section tests collection order, tube selection, technique, and the preanalytic errors that change results.
Order of draw
With a syringe or butterfly set, draw a discard tube first unless a blood culture bottle is the first tube. Then: blood culture (sterile), light blue citrate, serum tube with clot activator (SST/gold), heparin (green), EDTA (lavender), sodium fluoride/oxalate (gray). The order exists because additives carry over: EDTA contaminating a chemistry tube lowers calcium and raises potassium, and citrate carryover prolongs clotting times.
Technique
Keep the tourniquet on for less than one minute and release it before needle withdrawal; cleanse with 70% isopropyl and let it dry; insert bevel up at 15–30° into the antecubital fossa. Prefer the median cubital or cephalic vein and avoid the basilic site near the brachial artery, an arm with a dialysis fistula, a mastectomy arm with lymph node dissection, and areas of hematoma, edema, or infection.
Special situations
For capillary specimens, warm the site, puncture the side of the finger (not the tip), and wipe away the first drop. Blood cultures require 20–30 mL per set from two sites before antibiotics, with bottles inoculated before other tubes.
Worked example
A phlebotomist draws the EDTA tube and then the light blue citrate tube from the same venipuncture. The EDTA carryover chelates calcium in the citrate tube, prolonging the PT and aPTT and falsely lowering the calcium result. The correct action is to recollect the coagulation tube before the EDTA tube, following order of draw.
Traps that cost marks
Accepting an underfilled citrate tube. Hemolysis from a small-gauge needle or vigorous shaking, which raises potassium and LDH. Labeling tubes before the draw, which invites misidentification.