Which laboratory test is primarily affected by factor inhibitors?

Factor inhibitors specifically target coagulation factors in the blood, leading to a disruption in the normal clotting cascade. When factor inhibitors, such as those seen in certain bleeding disorders (for example, hemophilia or acquired factor inhibitors), are present, they primarily affect the intrinsic pathway of coagulation.

The activated partial thromboplastin time (APTT) test is designed to evaluate the functionality of this intrinsic pathway, where factors VIII, IX, XI, and XII play key roles. When factor inhibitors are present, they can significantly prolong the APTT, since the test relies on these factors for an appropriate clotting response. Therefore, the presence of factor inhibitors will lead to an abnormal APTT result, making it the laboratory test that is primarily affected.

In contrast, other tests such as fibrinogen levels, INR (which assesses the extrinsic pathway through prothrombin time), and PT are less sensitive to the effects of factor inhibitors on the intrinsic pathway. Thus, while they may reflect alterations in coagulation, they do not primarily assess the factors that are directly inhibited by factor inhibitors.