Proper venipuncture technique begins with thorough patient preparation and equipment selection. A tourniquet is applied three to four inches above the intended venipuncture site to engorge the veins with blood, making them easier to palpate and puncture. However, the tourniquet should remain in place no longer than one minute to prevent hemoconcentration, a relative increase in nonfilterable blood components that can falsely elevate values for large molecules and cellular elements. The patient's arm should be positioned in a straight, downward-sloping line from shoulder to wrist to promote venous filling. The patient should be instructed to make a fist only once, as repeated fist pumping can cause hemoconcentration and falsely elevated levels of potassium, ionized calcium, and lactate.
Needle selection depends on the patient's vein size and condition. A 21 to 22 gauge needle is typically used for routine adult venipuncture, while a 23 gauge needle, often attached to a winged infusion or butterfly set, is preferred for pediatric patients, elderly individuals, or those with small or fragile veins. Before use, the phlebotomist must verify the needle's expiration date, the integrity of the sterile packaging, and that the gauge is appropriate for the chosen vein. The needle should be inserted at a 15 to 30 degree angle with the bevel facing upward to allow smooth entry into the vein and minimize trauma. The winged infusion set provides additional control and flexibility for difficult draws but, as noted earlier, requires a discard tube before filling a citrate tube to clear air from the line.
After the site is selected, it should be cleaned using a circular motion from the center outward. Routine venipuncture sites are cleaned with 70% isopropyl alcohol, while blood culture collections or blood alcohol testing require chlorhexidine gluconate or povidone-iodine. For blood alcohol tests, alcohol antiseptics must be avoided entirely to prevent contamination of the specimen. The cleaned site must be allowed to air dry before puncture, as residual alcohol can cause stinging pain, hemolysis, and specimen contamination. In an evacuated tube system, the tube is pushed onto the needle inside the holder until the stopper is punctured, allowing the vacuum to draw blood into the tube in the proper order of draw.
Immediately after the needle is removed from the vein, firm pressure should be applied to the site with gauze. The patient should not be instructed to bend the arm, as this is less effective at stopping bleeding and can increase the risk of hematoma formation. If blood flow stops during the draw, the phlebotomist may make a minor adjustment to the needle position or angle, but if flow does not resume, the procedure should be discontinued and a new site selected. Throughout the procedure, the phlebotomist should monitor the patient for signs of complications such as vasovagal syncope, nerve injury, or hematoma formation, and respond appropriately to any patient reports of sharp, shooting, or electric shock-like pain.