Prothrombin Time

Book
In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan.
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Excerpt

Prothrombin time is one of several blood tests routinely used in clinical practice to evaluate the coagulation status of patients. More specifically, prothrombin time is used to evaluate the extrinsic and common pathways of coagulation, which helps detect deficiencies of factors II, V, VII, and X and low fibrinogen concentrations. Prothrombin time measures the time, in seconds, for plasma to clot after adding thromboplastin (a mixture of tissue factor, calcium, and phospholipid) to a patient's plasma sample.

Different preparations of thromboplastin reagents are available but can give different prothrombin time results even when using the same plasma. Due to this variability, the World Health Organization (WHO) introduced the international normalized ratio (INR), the standard reporting format for prothrombin time results. The INR represents the ratio of the patient's prothrombin time divided by a control prothrombin time value obtained using an international reference thromboplastin reagent developed by the WHO.

Standard laboratory coagulation-based testing has traditionally been used to obtain measurements of prothrombin time to ensure reliable results. Due to the high turnaround time (up to 90 minutes), point-of-care (POC) devices (approximately 5 minutes) are becoming more desirable. POC devices are of great value in emergency and operating room settings where clinical diagnosis and intervention are time-sensitive.

With increased prescribing of vitamin K-antagonists like warfarin, point-of-care devices have also been more convenient for patients and clinicians to monitor medication effectiveness. With point-of-care devices, monitoring anticoagulation therapy can take place at thrombosis centers, primary care provider offices, and even by the patients themselves. Although point-of-care devices have been shown to underestimate hemostatic abnormality, point-of-care devices are generally reliable in non-emergency settings.

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