What is the main additive in the gray top tube?

The main additive in the gray top tube is indeed sodium fluoride. Sodium fluoride serves as a glycolytic inhibitor, which means it prevents the breakdown of glucose in the blood sample, allowing for accurate measurement of glucose levels during tests. This is particularly important in situations where blood samples need to be processed later and prevents false results due to glucose metabolism by the blood cells. In addition, the gray top tube may also contain potassium oxalate, which acts as an anticoagulant, but sodium fluoride is the primary additive recognized for its role in preserving glucose concentration.

The other options—sodium citrate, EDTA, and heparin—are additives known for their use in different types of blood collection tubes but do not apply to the gray top tube. Sodium citrate is commonly found in light blue top tubes for coagulation studies, EDTA is used in lavender top tubes for hematology tests, and heparin is present in green top tubes for plasma chemistry tests. Each of these additives serves a specific purpose related to the type of analyses being performed, distinguishing their roles from those of sodium fluoride in the gray top tube.