Site selection: where to draw, where not to.

Median cubital first, basilic last, never through a fistula. The hierarchy and the do-not-draw list, with the exam-trap scenarios examiners love.

The default — and the rules around it

The median cubital vein in the antecubital fossa is the default for routine venipuncture. It's anchored, large, and sits over bone — which protects against accidental brachial artery puncture. When you have a clear median cubital, choose it.

Hierarchy of acceptable veins

• Median cubital (first choice). Center of the antecubital fossa. Best anchored, largest, lowest complication rate.
• Cephalic (second choice). Lateral side of the antecubital fossa. Slightly more mobile; can roll. Watch for the lateral antebrachial cutaneous nerve.
• Basilic (last choice). Medial side. Sits next to the brachial artery and median nerve — risk of arterial puncture or nerve damage. Use only when nothing else is accessible.

Where to never draw

• Same arm as a mastectomy. Lymphedema risk; the arm has impaired drainage and infection clearance.
• Above an IV line. The IV fluid will dilute or contaminate the sample. Below the IV is acceptable if the IV has been off for at least 2 minutes.
• Through a hematoma. The hemolyzed blood within the hematoma will skew chemistry results.
• Through a fistula or graft. Dialysis access. Never. Damage is permanent and life-threatening for the patient.
• Through scar tissue, burns, or tattoos. Distorted vasculature, possible analyte interference from tattoo pigment.
• Foot or ankle veins. Without a physician's order. Higher complication rate; reserved for last-resort orders only.

Standards reference: CLSI GP41-Ed7, Collection of Diagnostic Venous Blood Specimens, §5 (Site Selection and Patient Preparation). PhlebotomySkills.com is exam-preparation content. Not a degree, not for-credit coursework, and not affiliated with any certifying body.