Specimen Collection: Venipuncture and Micro-collection
Core concepts
Every draw begins with identification and order verification before any tube is selected. Know the order of draw for the evacuated tube system: blood culture (or sterile/yellow), then light-blue (sodium citrate), serum tubes with or without clot activator or gel (red, gold/SST), heparin tubes (green), EDTA tubes (lavender), and finally glycolytic inhibitor tubes (gray). The logic is additive carryover: clot activators and EDTA must not contaminate coagulation testing, and heparin interferes with many assays.
Venipuncture technique questions focus on site selection, tourniquet time, and needle angle. The antecubital fossa is preferred; the median cubital vein is first choice, cephalic second, basilic last because of proximity to the brachial artery and median nerve. Apply the tourniquet 3–4 inches above the site for no more than one minute; release before removing the needle. Insert at roughly 15–30 degrees, bevel up. Never draw above an IV site or from an arm with a dialysis fistula, mastectomy-related lymphedema, or active infection.
Micro-collection (capillary/dermal puncture) is chosen for infants, patients with fragile or inaccessible veins, and point-of-care testing. Warm the site, cleanse, and puncture the lateral or medial plantar surface of the heel in infants (never the center or posterior curve) or the side of a fingertip in adults. Wipe away the first drop because it contains tissue fluid, then let blood flow freely; avoid excessive squeezing, which hemolyzes the sample and dilutes it with interstitial fluid.
Traps and rule of thumb
Common distractors: choosing the basilic vein as "first choice," leaving the tourniquet on for two minutes, or drawing a light-blue tube after a green tube. Rule of thumb: if a question asks what to do when a tube fills poorly, the answer is usually to remove the tourniquet, reposition, and redraw with a fresh needle rather than to milk the site or probe with the needle.