Which clotting factor is NOT measured by PT and APTT tests?

The correct answer is Factor XIII because it is not assessed in standard Prothrombin Time (PT) or Activated Partial Thromboplastin Time (APTT) tests. PT and APTT are commonly used laboratory tests that measure the functionality of specific clotting factors involved in the coagulation cascade. PT primarily evaluates the extrinsic and common pathways of coagulation and assesses factors such as I (fibrinogen), II (prothrombin), V, VII, and X. APTT evaluates the intrinsic and common pathways, measuring factors I, II, V, VIII, IX, X, XI, and XII.

Factor XIII, also known as fibrin-stabilizing factor, plays a role in the final stages of coagulation, where it helps to stabilize the fibrin clot by cross-linking fibrin strands. Because its function is related to the stabilization phase rather than the initial formation of the fibrin clot, it is typically assessed through other specific tests, such as a factor XIII assay, rather than PT or APTT, which do not provide information about its activity.