Specimen Collection
Specimen Collection tests order of draw, tube additives, needle and device selection, venipuncture and capillary technique, and prevention of contamination and hemolysis. Expect questions that show a collection sequence or a poorly filled tube and ask what is wrong.
Order of draw
For routine venipuncture with multiple tubes, follow the CLSI sequence: blood culture first if ordered, then sodium citrate light blue, serum red or gold, heparin green, EDTA lavender, and fluoride-oxalate gray. The sequence prevents additive carryover that could alter coagulation or chemistry results. Plastic tubes follow the same order.
Technique and tubes
- Use a 21-gauge needle for routine adult draws and a 23-gauge for small or fragile veins; a winged set helps with hand veins. A needle that is too small can hemolyze cells.
- Fill tubes to the nominal volume. Citrate tubes must reach the fill line to preserve the 9:1 blood-to-additive ratio; short draws are rejected.
- Invert additive tubes gently per manufacturer instructions, usually 3-10 times; never shake. Allow clot tubes to stand upright.
- For capillary collection, warm the site, wipe away the first drop, and avoid squeezing. Follow the lab manual for capillary order; CLSI generally places blood gas first, then EDTA, other additive, and serum tubes.
Worked example
A light blue tube fills only halfway. Adding blood from a second blue tube is unacceptable because it does not correct the anticoagulant ratio. The tube is discarded and the specimen is recollected, because underfilling can make coagulation times falsely prolonged.
Traps that cost marks
- Drawing EDTA before citrate or serum.
- Accepting a short-filled citrate tube.
- Probing repeatedly or using a small needle, causing hemolysis.
- Labeling tubes before the draw instead of at the bedside.