What is the main reason for using buffered sodium citrate in the black top tube?

Buffered sodium citrate is primarily used in the black top tube to form calcium salts, which are essential in preventing blood from clotting during laboratory tests. This anticoagulant works by binding calcium ions in the blood, which are necessary for the coagulation cascade. By sequestering calcium, the citrated blood remains in a liquid state, allowing for accurate testing of the blood's components without interference from clotting factors.

This characteristic is essential for various hematological tests, including the erythrocyte sedimentation rate (ESR), which is measured in the black top tube. However, the primary purpose of using buffered sodium citrate is to create an environment where calcium is bound, thereby effectively inhibiting the clotting process itself. This ensures that all necessary tests can be performed on blood samples without the risk of clots compromising the results.