What the colours of vacuum blood tubes mean
Red, yellow, blue, pink and black caps — which tube for which test, and the order of draw.
The cap colour of a vacuum tube identifies the additive inside. The wrong tube makes the sample unusable, so this table belongs somewhere visible in the phlebotomy room.
Red — clot activator, plain serum. Used for biochemistry, serology and hormone assays. Yellow — clot activator with gel: after centrifugation the gel separates serum from cells and extends how long the sample keeps.
Blue — sodium citrate 3.2%. For coagulation tests (PT, APTT, fibrinogen). Fill level is critical here: the blood-to-anticoagulant ratio must be 9:1, and an underfilled tube distorts the result.
Pink or lavender — K2/K3 EDTA. For full blood count and HbA1c. Black — citrate 4NC, prepared at a separate ratio for ESR determination.
Order of draw. To prevent additive carry-over between tubes, the international order is: blood culture first, then citrate (blue), then serum (red/yellow), then heparin, then EDTA (pink), and the glucose tube last. Breaking this order shows up most clearly in coagulation results.