PTT vs PT/INR: Distinct Tests for Different Coagulation Pathways
PTT (Partial Thromboplastin Time) and PT/INR (Prothrombin Time/International Normalized Ratio) are fundamentally different coagulation tests that evaluate separate pathways of the clotting cascade and are used to monitor different anticoagulants.
What Each Test Measures
PTT (Partial Thromboplastin Time)
- PTT evaluates the intrinsic and common coagulation pathways, specifically assessing factors I, II, V, VIII, IX, X, XI, and XII 1, 2
- PTT is the primary test for monitoring unfractionated heparin therapy, measuring the anticoagulant effect of direct thrombin inhibitors and UFH 1
- PTT does not measure the effect of low molecular weight heparin (LMWH), which requires anti-Factor Xa levels instead 1
PT/INR (Prothrombin Time/International Normalized Ratio)
- PT/INR evaluates the extrinsic and common coagulation pathways, including factors II, V, VII, X, and fibrinogen 3
- PT/INR is specifically designed and validated only for monitoring vitamin K antagonist therapy (warfarin), not as a general bleeding predictor 2, 4
- The INR standardizes PT results using the international sensitivity index (ISI) of the thromboplastin reagent, allowing comparison across different laboratories 4, 5
Clinical Applications: When to Use Each Test
Use PTT for:
- Monitoring unfractionated heparin therapy with target aPTT 1.5-2.0 times control 6
- Evaluating suspected hemophilia A (Factor VIII deficiency) or hemophilia B (Factor IX deficiency) 6
- Assessing patients on direct thrombin inhibitors like argatroban, lepirudin, or bivalirudin 1, 7
- Screening for lupus anticoagulant or other intrinsic pathway abnormalities 3
Use PT/INR for:
- Monitoring warfarin therapy exclusively, with therapeutic targets typically INR 2.0-3.0 1, 4
- PT/INR should be determined daily after warfarin initiation until results stabilize in therapeutic range 4
- Intervals between subsequent PT/INR determinations range from one to four weeks once stable dosage achieved 4
Critical Pitfalls to Avoid
Do NOT Order Both Tests Routinely
- Ordering PT and PTT together is inappropriate in most clinical scenarios and represents significant overuse 8, 9
- A study found PT and PTT were ordered together in all 50 patients reviewed, despite lack of indication, resulting in 232 unnecessary tests 8
- Order PTT only for heparin monitoring; order PT/INR only for warfarin monitoring 8
INR Has Limited Utility Outside Warfarin Monitoring
- INR was designed specifically for vitamin K antagonist therapy and should not be used as a universal bleeding predictor 2
- A systematic review found weak or no association between pre-procedural INR and bleeding in 78 of 79 studies assessed 2
- INR is a poor predictor of bleeding risk in liver disease, trauma, or patients on direct oral anticoagulants 3, 2
PTT Limitations in Special Populations
- In infants, PTT does not correspond to anti-Factor Xa levels due to developmental hemostasis 1
- PTT shows poor correlation with heparin levels in children and may be unreliable with elevated Factor VIII, fibrinogen, or presence of antiphospholipid antibodies 1
- Baseline PTT abnormalities can confound monitoring of heparin or direct thrombin inhibitors, leading to anticoagulant failure 10
Monitoring During Combination Therapy
Heparin-Warfarin Overlap
- When transitioning from heparin to warfarin, overlap therapy for 4-5 days until therapeutic INR achieved 4
- Draw blood for PT/INR at least 5 hours after last IV heparin bolus, 4 hours after stopping continuous infusion, or 24 hours after subcutaneous heparin 4
- Warfarin may increase aPTT even without heparin present; severe elevation (>50 seconds) with therapeutic INR indicates increased postoperative hemorrhage risk 4
Alternative Monitoring Methods
- For patients with baseline coagulation abnormalities, consider anticoagulants not monitored by global assays (fondaparinux, danaparoid) or obtain specific drug levels (anti-Factor Xa, DTI levels) 10
- Thromboelastography (TEG) and rotational thromboelastometry (ROTEM) provide more comprehensive coagulation assessment, especially in trauma or massive bleeding 1, 2, 9