Expected PTT on Coumadin (Warfarin)
Coumadin (warfarin) does not require PTT monitoring and typically does not significantly affect PTT values in therapeutic dosing—the primary monitoring test is INR, with a therapeutic target of 2.0-3.0 for most indications. 1, 2
Why PTT is Not Used to Monitor Warfarin
Warfarin primarily affects the extrinsic and common coagulation pathways (factors II, VII, and X), which are measured by PT/INR, not the intrinsic pathway measured by PTT 3, 4
The FDA-approved warfarin label explicitly states that PT/INR should be used for monitoring, with no mention of PTT as a monitoring parameter for warfarin therapy 2
PTT measures the intrinsic and common pathways (factors VIII, IX, XI, XII) and is primarily used to monitor heparin therapy, not warfarin 4, 5
What Happens to PTT on Warfarin
Warfarin may cause mild PTT elevation even in the absence of heparin, but this is not clinically significant for monitoring purposes 2
A severe elevation in PTT (>50 seconds) with a therapeutic INR has been identified as an indication of increased risk of postoperative hemorrhage, suggesting this combination warrants clinical attention 2
During the transition from heparin to warfarin, PTT interference is of minimal clinical significance once warfarin reaches therapeutic levels 2
Critical Monitoring Parameters for Warfarin
INR is the standardized test for warfarin monitoring, with therapeutic range 2.0-3.0 for most indications (atrial fibrillation, venous thromboembolism) and 2.5-3.5 for mechanical heart valves 1, 4
PT should be checked daily after initial dosing until INR stabilizes in therapeutic range, then intervals of 1-4 weeks once stable 1, 2
INR >4.0 provides no additional therapeutic benefit and significantly increases bleeding risk 2
Common Clinical Pitfall
When transitioning from heparin to warfarin, blood for PT/INR should be drawn at specific times to avoid heparin interference: at least 5 hours after the last IV heparin bolus, 4 hours after stopping continuous IV heparin infusion, or 24 hours after subcutaneous heparin injection 2. This timing ensures accurate INR measurement without heparin contamination affecting the result.