What laboratory test is used to screen for activated protein C resistance?

To determine activated protein C resistance, the appropriate laboratory test involves a modified activated partial thromboplastin time (APTT) assay that is conducted both with and without activated protein C. This test is specifically designed to assess how effectively activated protein C can manage the coagulation process. In patients with activated protein C resistance, the presence of activated protein C does not prolong the clotting time sufficiently compared to normal cases.

The rationale behind using modified APTT is that it allows clinicians to observe the functional activity of the coagulation cascade when activated protein C is introduced. When activated protein C is functioning normally, it should promote the breakdown of factors Va and VIIIa, leading to a longer clotting time. If resistance is present, the clotting time remains relatively short, indicating that the anticoagulant effects of activated protein C are impaired, often due to mutations like the Factor V Leiden mutation.

In contrast, the other testing options, while useful in various contexts, do not specifically evaluate the interaction between activated protein C and the coagulation factors involved. Mixing studies are generally used to investigate the presence of coagulation factor inhibitors or deficiencies, rather than specifically assessing the functionality of activated protein C. Therefore, the choice that directly evaluates activated protein C resistance is the