A patient with hemophilia needs a diagnostic coagulation test. Which is the best test to use?

Hemophilia A and B are deficiencies of clotting factors VIII and IX respectively, both part of the intrinsic coagulation pathway, which is specifically assessed by PTT (Partial Thromboplastin Time) — PTT is prolonged in hemophilia. PT/INR assess the extrinsic/common pathway (and are used to monitor warfarin), and platelet count assesses platelet number, not clotting factor function — none of these are as specific to hemophilia as PTT.