The APTT test is sensitive to deficiencies of which clotting factor?

The APTT (Activated Partial Thromboplastin Time) test is primarily sensitive to deficiencies in factors involved in the intrinsic and common pathways of the coagulation cascade. Among the factors affected, Factor X plays a significant role as it is part of the common pathway that leads to the formation of fibrin.

When Factor X is deficient, the surface activation leading to the clotting cascade is impaired, resulting in a prolonged APTT. This test is crucial for assessing clotting disorders and monitoring patients on anticoagulant therapy, particularly those using heparin, which also affects factors in the intrinsic pathway.

Factors such as Factor VII, which is part of the extrinsic pathway, and calcium, which is necessary for the coagulation process, do not rely on the APTT for their sensitivity; rather, they are assessed through other tests. Additionally, PF3 (Platelet Factor 3) is a phospholipid component essential for clotting but is not a clotting factor itself and does not directly cause a prolonged APTT when deficient. Therefore, the sensitivity of the APTT test specifically points to deficiencies in Factor X, confirming that it is the correct choice in this context.