What is the additive in a blue top tube?

A blue top tube contains sodium citrate as its additive. Sodium citrate is an anticoagulant that is primarily used for coagulation studies, as it works by binding calcium ions in the blood, which are necessary for clotting. This allows for the collection of plasma for various tests related to blood clotting factors, such as prothrombin time (PT) and activated partial thromboplastin time (aPTT).

The other options do not represent the correct additive for a blue top tube. Serum separator additives are found in gold or tiger top tubes, which are intended for serum collection and do not serve the same purpose as sodium citrate. EDTA is the additive found in lavender top tubes, used mainly for hematology tests as it preserves cell morphology and prevents clotting by chelating calcium, but it is not used in blue top tubes. Thus, the key additive in a blue top tube is indeed sodium citrate, making it essential for performing specific laboratory analyses related to coagulation.