Tube colors, additives, and what each is for.

Six tubes, six purposes, three retake-trap pairings. The Phlebotomy 101 lesson that pays for itself by saving you the glucose-on-Green miss.

The six tubes you'll see on the exam

• Yellow / Black (SPS). Sodium polyanethol sulfonate. Used for blood cultures. Drawn first to keep the sample sterile.
• Light Blue (Sodium Citrate, 3.2%). Coagulation tests (PT, INR, PTT, fibrinogen). Fill exactly to the line — the 9:1 blood-to-anticoagulant ratio is non-negotiable.
• Red or Gold (Clot activator / SST gel). Chemistry, lipid panel, TSH, and most serum tests. Invert gently 5 times; let stand upright 30-60 minutes before centrifugation.
• Green (Lithium or Sodium Heparin). Stat chemistry, ammonia, stat electrolytes. Heparin works by activating antithrombin, blocking the coagulation cascade upstream.
• Lavender (EDTA). CBC, blood type, HbA1c, reticulocyte count. EDTA chelates calcium; preserves cellular morphology better than other anticoagulants.
• Gray (Sodium Fluoride / Potassium Oxalate). Fasting glucose, lactate, blood alcohol. Fluoride inhibits glycolysis — that's why glucose belongs here, not on Green.

Pink (EDTA) is sometimes seen as a 7th tube — it's chemically identical to Lavender but is reserved for blood-bank work (type and crossmatch) because it carries strict labeling requirements. Royal Blue is a specialty trace-element tube (free of contamination from the additives in standard tubes); used for toxicology and trace-metal testing.

Standards reference: CLSI GP41-Ed7, Collection of Diagnostic Venous Blood Specimens, §6.5 (Additives and Tube Selection). PhlebotomySkills.com is exam-preparation content. Not a degree, not for-credit coursework, and not affiliated with any certifying body.