What does APTT measure in terms of blood coagulation factors?

APTT, or activated partial thromboplastin time, is a laboratory test primarily used to evaluate the intrinsic pathway of the coagulation cascade. Specifically, it assesses the activity of several clotting factors involved in this pathway, including factors VIII, IX, XI, and XII.

The correct identification of factors 8, 9, 11, and 12 is linked to the clinical significance of APTT testing; it is particularly useful for monitoring patients who are on anticoagulant therapy, such as heparin, and for evaluating bleeding disorders like hemophilia. Hemophilia A is associated with a deficiency in factor VIII, while hemophilia B relates to a deficiency in factor IX.

Factors 5, 7, and 10 are not measured by the APTT test, as these factors are primarily associated with the extrinsic pathway and common pathway of coagulation, which are evaluated using prothrombin time (PT). Similarly, factors 2, 3, and 4 do not pertain to the intrinsic pathway measured by APTT, with factor 3 being tissue factor (not measured in this test), and factors 1 (fibrinogen) and 6 being unrelated in context