Which of the following indirect tests is insensitive to LMWH effects?

The correct choice highlights that the activated partial thromboplastin time (APTT) is the indirect test that is insensitive to the effects of low molecular weight heparin (LMWH).

APTT primarily measures the intrinsic and common pathways of coagulation, specifically the activities of factors VIII, IX, XI, and XII, as well as factors I, II, V, and X. While LMWH does affect these pathways, the APTT test has a limited ability to detect the anticoagulant effects of LMWH due to its relatively insensitivity. This means that even if a patient is receiving LMWH, the APTT may remain within normal limits, leading to potential underestimation of the anticoagulant effect.

In contrast, other tests such as anti-Xa assays are specifically designed to measure the activity of LMWH by quantifying the levels of antifactor Xa activity, thereby providing a direct assessment of LMWH efficacy. Prothrombin time also reflects the extrinsic and common pathways, and it can be influenced by vitamin K antagonists rather than LMWH directly. The activated clotting time, meanwhile, is used primarily for monitoring heparin therapy and can show sensitivity to LMWH.

Thus, the insensitivity of the