When to monitor Prothrombin Time (PT) versus Partial Thromboplastin Time (PTT) in anticoagulant therapy?

From the Guidelines

Prothrombin Time (PT) should be monitored for patients on vitamin K antagonists like warfarin, while Partial Thromboplastin Time (PTT) is used for monitoring unfractionated heparin therapy. For warfarin, PT is typically reported as International Normalized Ratio (INR) with target ranges of 2.0-3.0 for most indications or 2.5-3.5 for mechanical heart valves. INR should be checked frequently when initiating therapy (every 1-2 days), then weekly until stable, and eventually monthly for stable patients. For unfractionated heparin, PTT should be monitored 6 hours after initiation and dose changes, with a typical target of 1.5-2.5 times the normal control value (approximately 60-80 seconds, though this varies by laboratory) 1. Low molecular weight heparins (like enoxaparin) and direct oral anticoagulants (like apixaban, rivaroxaban) generally don't require routine coagulation monitoring. PT measures the extrinsic and common coagulation pathways affected by warfarin's inhibition of vitamin K-dependent clotting factors, while PTT assesses the intrinsic and common pathways affected by heparin's enhancement of antithrombin activity. Monitoring these values helps prevent both thrombotic events from undertreatment and bleeding complications from overtreatment.

Key Considerations

• The INR has been used for years and proved to be effective/safe for monitoring patients on VKA 1.
• Patients who are positive for lupus anticoagulants (LA) and/or antiphospholipid antibodies and have had arterial and/or venous thromboembolism are candidates for long-term anticoagulation with VKA 1.
• The PT-INR measured with the vast majority of commercial thromboplastins can be safely used to monitor LA-positive patients on VKA, keeping in mind that there may be occasional patients for whom the INR is affected by LA 1.
• The need for the INR stems from the evidence that PT results obtained with commercial thromboplastins are varied owing to their different sensitivity to the coagulation defect induced by VKA 1.

Monitoring Frequency

• INR should be checked frequently when initiating warfarin therapy (every 1-2 days), then weekly until stable, and eventually monthly for stable patients.
• For unfractionated heparin, PTT should be monitored 6 hours after initiation and dose changes.

Target Ranges

• INR target ranges are typically 2.0-3.0 for most indications or 2.5-3.5 for mechanical heart valves.
• PTT target is typically 1.5-2.5 times the normal control value (approximately 60-80 seconds, though this varies by laboratory).

From the FDA Drug Label

The PT reflects the depression of vitamin K dependent Factors VII, X and II. Warfarin sodium tablets may increase the activated partial thromboplastin time (aPTT) test, even in the absence of heparin Heparin sodium may prolong the one-stage prothrombin time.

Monitoring Parameters:

• Prothrombin Time (PT): should be monitored in patients on warfarin therapy to assess the depression of vitamin K dependent factors VII, X, and II.
• Partial Thromboplastin Time (PTT): should be monitored in patients on heparin therapy to assess the anticoagulant effect of heparin.
• International Normalized Ratio (INR): should be monitored in patients on warfarin therapy to provide a common basis for communication of PT results and interpretations of therapeutic ranges.

Key Points:

• PT is used to monitor warfarin therapy.
• PTT is used to monitor heparin therapy.
• INR is used to monitor warfarin therapy and provide a standardized measure of anticoagulation. 2 3

From the Research

Monitoring Anticoagulant Therapy

• Prothrombin Time (PT) is used to measure the effect of warfarin, whereas the partial thromboplastin time (PTT) measures the therapeutic effect of unfractionated heparin 4.
• Low molecular weight heparin (LMWH) does not require routine monitoring 4.

Standardization of Coagulation Tests

• The standardization of PT has been successfully implemented, while the standardization of activated partial thromboplastin time (APTT) is still underway 5.
• The International Normalized Ratio (INR) system is used to standardize the reporting of PT, and it is being adopted by an increasing number of hospitals in many countries 5, 6.

Comparison of PT and PTT

• PT is a common method for monitoring oral anticoagulant therapy, while PTT is used to monitor the therapeutic effect of unfractionated heparin 5, 4.
• The use of PT and PTT can be optimized by selecting sensitive thromboplastin with low ISI values and by using a mean normal prothrombin time (MNPT) obtained with the coagulometer to derive the prothrombin ratio 5.

Clinical Implications

• The INR system has been adopted in many countries for anticoagulant therapy, and it is widely held that this is the best available system for standardization of PT for monitoring oral anticoagulants 6.
• Fiix-prothrombin time (PT) monitoring of warfarin, which measures factor (F) II and X, is effective and reflects the anticoagulation level better than the standard prothrombin time 7.