Which laboratory test is MOST appropriate to evaluate a patient with hemophilia A before an invasive dental procedure?

Correct answer: C. Partial thromboplastin time (PTT/aPTT)

• A. Bleeding time reflects platelet function and vascular integrity; it is normal in hemophilia and is largely obsolete as a screening test.
• B. Platelet count is normal in hemophilia — the defect is in the coagulation cascade, not in platelet number.
• C. Correct. Hemophilia A is a deficiency of FACTOR VIII, a component of the INTRINSIC pathway — which is assessed by the partial thromboplastin time (PTT/aPTT). The aPTT is prolonged; all other routine coagulation screens are normal.
• D. PT (and INR) measures the EXTRINSIC/common pathway (factor VII) and is used to monitor warfarin. It is NORMAL in hemophilia.

Note: Mnemonic — PT/INR = extrinsic = warfarin. PTT/aPTT = intrinsic = heparin and hemophilia A/B. Hemophilia B (Christmas disease) is factor IX deficiency and also prolongs the aPTT.