What is the primary use of the sodium citrate in a blue top tube?

Sodium citrate is primarily used as an anticoagulant in blood collection tubes, such as the blue top tube. Its main function is to prevent the coagulation of blood by chelating calcium, which is a vital component in the clotting cascade. When sodium citrate is added to the blood sample, it binds to the calcium ions, thus inhibiting the blood from clotting and allowing for the accurate analysis of plasma components. This anticoagulant property makes it particularly useful in tests that require the measurement of clotting factors, such as prothrombin time (PT) or activated partial thromboplastin time (aPTT), ensuring that the sample remains uncoagulated until analysis is complete.

In contrast, choices regarding promoting or measuring clotting are not applicable to sodium citrate's role, as its mechanism is specifically designed to prevent clot formation rather than facilitating it. Similarly, options related to separating serum do not pertain to sodium citrate's function in the blue top tube, as the separation occurs after the blood has clotted (if a clotting tube is used) or not at all when sodium citrate is present. Lastly, while sodium citrate may indirectly affect glucose measurements due to its anticoagulant properties, it is not primarily