Results Warfarin markedly affected APTT; for each increase of 1.0 in the international normalized ratio, the APTT increased 16 seconds (95% confidence interval, 10-22 seconds). The effects of warfarin and heparin on APTT were additive.

Does warfarin increase PTT?

Warfarin (Coumadin ^®) anticoagulation therapy—the PTT is not used to monitor warfarin therapy, but PTT may be prolonged by warfarin at high dose.

Does warfarin increase PT and APTT?

Warfarin typically prolongs the PT alone, but at high levels warfarin can prolong both tests. Heparin typically prolongs the aPTT alone (because PT reagents contain heparin-binding agents that block heparin effect), but at high levels heparin can prolong both tests.