Most blood collection tubes contain an additive that either accelerates clotting of the blood (clot activator) or prevents the blood from clotting (anticoagulant). A tube that contains a clot activator will produce a serum sample when the blood is separated by centrifugation and a tube that contains an anticoagulant will produce a plasma sample after centrifugation. Some tests require the use of serum, some require plasma, and other tests require anticoagulated whole blood (Table 1).
Table 1. The most commonly used blood collection tubes.
|Tube cap color
|Function of Additive
|Common laboratory tests
|3.2% Sodium citrate
|Prevents blood from clotting by binding calcium
Red or gold (mottled or "tiger" top used with some tubes)
|Serum tube with or without clot activator or gel
|Clot activator promotes blood clotting with glass or silica particles. Gel separates serum from cells.
|Chemistry, serology, immunology
|Sodium or lithium heparin with or without gel
|Prevents clotting by inhibiting thrombin and thromboplastin
|Stat and routine chemistry
Lavender or pink
|Prevents clotting by binding calcium
|Hematology and blood bank
|Sodium fluoride, and sodium or potassium oxalate
|Fluoride inhibits glycolysis, and oxalate prevents clotting by precipitating calcium.
| Glucose (especially when testing will be delayed), blood alcohol, lactic acid