From the Guidelines
Prothrombin time (PT) indicates how quickly your blood clots, specifically measuring the function of the extrinsic and common pathways of the coagulation cascade, and is validated for assessing coagulation status in patients on vitamin K antagonist (VKA) therapy. It evaluates the activity of clotting factors I (fibrinogen), II (prothrombin), V, VII, and X. A prolonged PT suggests that blood is taking longer than normal to clot, which may indicate conditions such as liver disease, vitamin K deficiency, or the effect of anticoagulant medications like warfarin 1.
Key Points to Consider
- PT results are often reported alongside the International Normalized Ratio (INR), which standardizes results across different laboratories 1.
- Normal PT ranges are typically 10-13 seconds, though this varies by laboratory.
- For patients on warfarin therapy, the target INR usually ranges from 2.0-3.0 for most conditions, with higher targets (2.5-3.5) for mechanical heart valves.
- The INR was developed to standardize the PT across different thromboplastins and minimize variability in PT results between laboratories, and its use has improved the safety of oral anticoagulant therapy and its ease of monitoring 1.
Clinical Implications
- Regular PT/INR monitoring is essential for patients on warfarin to ensure they remain within therapeutic range and avoid bleeding complications or clot formation.
- The PT and INR are not designed for general screening of coagulopathy, and their use for this purpose lacks face validity and clinical evidence 1.
- Attempts to correct incidentally abnormal INRs can lead to high-volume plasma transfusions with enormous costs, definite harms, and dubious gains 1.
From the FDA Drug Label
24The PT should be determined daily after the administration of the initial dose until PT/INR results stabilize in the therapeutic range. PT/INR determination is recommended just prior to any dental or surgical procedure. When warfarin sodium tablets have produced the desired PT/INR or prothrombin activity, heparin may be discontinued.
The prothrombin time (PT) indicates the blood's clotting ability, and the International Normalized Ratio (INR) is a calculation based on the PT. The PT/INR is used to monitor patients on warfarin therapy to ensure they are within the therapeutic range.
- The PT/INR is used to determine the effectiveness of warfarin therapy.
- It is recommended to determine the PT/INR before any dental or surgical procedure.
- The PT/INR is used to guide the management of warfarin therapy, including adjusting the dosage and determining when to stop or start therapy 2.
From the Research
Prothrombin Time (PT) Indications
- The prothrombin time (PT) is a coagulation test that represents the most commonly used coagulation test in clinical laboratories 3
- PT is used to diagnose the risk of bleeding and to monitor oral anticoagulation therapy 4
- It is mathematically converted to the international normalized ratio (INR) for use in monitoring anticoagulant therapy with vitamin K antagonists such as warfarin 3
Relationship between PT and INR
- The INR is created using two major PT 'correction factors', namely the mean normal PT (MNPT) and the international sensitivity index (ISI) 3
- The INR provides different information about the coagulation status in patients with disseminated intravascular coagulation than in patients receiving warfarin 5
- Equivalent INR values can be associated with distinct coagulation profiles in different patient groups 5
Clinical Applications of PT/INR
- PT/INR is used to monitor warfarin-treated patients and to diagnose coagulation abnormalities in patients with sepsis 5
- Vitamin K administration is effective and safe for the management of vitamin K-deficient coagulopathy in critically ill pediatric patients, with a decrease in PT/INR after vitamin K administration 6
- The use of PT/INR is crucial in the management of warfarin-associated coagulopathy, with guidelines for vitamin K replacement and factor replacement for immediate correction of the coagulopathy 7