Which laboratory test is affected by heparin therapy?

Heparin therapy significantly impacts thrombin time, making it the correct option. Thrombin time is a measure of how long it takes for a clot to form in plasma after adding thrombin. Heparin, an anticoagulant, inhibits the formation of thrombin and thus prolongs the thrombin time. When a patient is on heparin therapy, the test may show an extended time due to the drug’s interference with the coagulation cascade, specifically the conversion of fibrinogen to fibrin, which is what thrombin normally facilitates.

In contrast, the fibrinogen assay, protein C assay, and protein S assay are generally unaffected by heparin in the same direct manner. While these tests may reflect changes in coagulation pathways, their specific measurements do not rely on thrombin activity to the extent that thrombin time does, allowing them to remain reliable even in patients receiving heparin. This is why thrombin time serves as a key indicator of heparin therapy's influence on the coagulation process.