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Activated partial thromboplastin time measures the intrinsic pathway. The intrinsic pathway starts with contact activation of factors XII, XI, IX, and VIII, and then feeds into the common pathway (factors X, V, II, and I). The test uses a phospholipid-containing reagent and an activator to trigger this cascade and then records the time it takes for a fibrin clot to form after adding calcium. Prolongation can indicate deficiencies in intrinsic pathway factors (like VIII, IX, XI, XII) or inhibition (such as heparin therapy), making it the best choice to assess intrinsic coagulation.

PT and INR assess the extrinsic pathway, which involves tissue factor and factor VII, not the intrinsic factors. D-dimer isn’t a test of coagulation timing at all; it reflects fibrin degradation and is used to assess clot breakdown.