Which additive is found in the black top tube?

The black top tube contains buffered sodium citrate as its additive, which is used primarily for coagulation studies. Buffered sodium citrate acts as an anticoagulant by binding calcium ions in the blood, thereby preventing clotting during the testing process. This characteristic is crucial for tests such as the erythrocyte sedimentation rate (ESR) as it allows for an accurate assessment of blood's ability to clot. Buffered sodium citrate is specifically formulated to maintain a stable pH and prevent the effects of varying blood pH on the coagulation tests being performed. This makes the black top tube particularly important for specific laboratory analyses requiring precise and reliable coagulation measurement.

The other options do not align with the purpose of the black top tube: sodium heparin is found in green top tubes and is also an anticoagulant but functions differently; potassium EDTA is used in lavender top tubes primarily for hematology tests; and a broth mixture does not correspond to the additives commonly associated with phlebotomy tubes used for blood collection.