Which tube contains EDTA and is most commonly used for complete blood counts (CBC)?

• A. Lavender top
• B. Gray top
• C. Green top
• D. Light blue top

Answer: (A)

For a complete blood count, you need a tube that stops the blood from clotting without altering the size or appearance of the blood cells. EDTA does this effectively by binding calcium, which is needed for clotting, so the cells stay intact and can be counted accurately by automated analyzers. The lavender-top tube is the standard container for CBC because it contains EDTA (commonly K2EDTA or K3EDTA), providing reliable preservation of white cells, red cells, and platelets for accurate counts and indices.

Other tubes are used for different tests. A gray-top tube contains fluoride and oxalate, mainly for glucose testing and glycolysis inhibition; a green-top tube uses heparin for plasma chemistry studies; a light-blue top contains sodium citrate for coagulation testing like PT and aPTT. These are not suitable for CBC because they either don’t preserve cells in the same way or can interfere with cell morphology and counts.