What condition is primarily tested for using the light blue top tube?

The light blue top tube is primarily used for testing prothrombin time (PT) and international normalized ratio (INR). These tests assess the blood's ability to clot properly and are crucial in monitoring patients on anticoagulant therapy, such as warfarin. The anticoagulant present in the light blue tube is sodium citrate, which binds calcium and ensures that the blood remains in a liquid state for the duration of the testing process. This is essential for accurate assessment of the coagulation pathways.

The other options involve tests that do not utilize the light blue top tube. Complete blood counts (CBC) are typically performed using a lavender or purple top tube that contains EDTA as an anticoagulant. Blood cultures are processed in specialized bottles rather than conventional vacuum tubes and are meant for detecting microbial infections. Thyroid function tests usually require serum specimens collected in red or gold top tubes that allow the blood to clot before the serum is separated for analysis.