Phlebotomy

Diagnostic phlebotomy uses two main techniques. Venipuncture collects a larger volume from a vein and is the standard approach for most laboratory work. Capillary or dermal puncture collects a small sample from a fingerstick or, in infants under one year, a heelstick on the medial or lateral plantar surface.

For routine venipuncture the antecubital fossa is the preferred site. The median cubital vein is the first choice because it is large, well anchored, and sits away from major nerves and arteries; the cephalic vein is the second choice; and the basilic vein is selected last because of its proximity to the brachial artery and median nerve. The needle enters bevel up at a shallow angle, and the tourniquet should not remain in place beyond about a minute, since prolonged constriction causes hemoconcentration and skews results.

When multiple evacuated tubes are drawn, the order of draw prevents additive from one tube carrying over and contaminating the next. The standard sequence is blood culture bottles, then the light blue sodium citrate tube, then serum tubes (plain red or gel separator), then green heparin tubes, then lavender EDTA tubes, and finally gray tubes containing sodium fluoride and potassium oxalate. Correct sequence matters clinically: EDTA carryover into a chemistry tube, for example, falsely lowers calcium and raises potassium.

Recognized complications include hematoma from incomplete vein puncture or inadequate pressure afterward, hemolysis from an undersized needle or vigorous tube mixing, nerve injury from probing, syncope, and iatrogenic anemia in patients drawn repeatedly. Standard precautions, two-identifier patient verification, and labeling at the bedside are core safeguards. Sites are avoided on an arm with an active IV line above the draw site or on the side of a prior mastectomy.

Therapeutic phlebotomy removes blood as treatment rather than for testing. It is a mainstay in polycythemia vera to reduce red cell mass and blood viscosity, and it is used in porphyria cutanea tarda and hereditary hemochromatosis to lower iron stores.

The CCMA exam covers phlebotomy thoroughly in its blood collection domain, testing site selection, order of draw, and the recognition and prevention of venipuncture complications. USMLE Step 1 encounters the term from the opposite direction, as the therapeutic intervention associated with polycythemia vera and with the porphyrias.