How Does a PT-INR Test Scale Work?


Quick Answer

A prothrombin time, or international normalized ratio, test measures the amount of time it takes for a patient’s blood to begin clotting in a laboratory setting, explains WebMD. The prothrombin time scale compares the patient's rate of clotting to a standard range of 11 to 13 seconds. Because the precise methods for measuring this vary between labs, the international normalized ratio standardizes the measurements.

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Full Answer

Patients with a high prothrombin time may have low levels of natural blood-clotting agents, such as factors I, II, V, VII or X, according to WebMD. Insufficient vitamin K intake can also lead to a high prothrombin time, as can some medical conditions, such as liver disease, liver injury or disseminated intravascular coagulation. Prescription medications, such as blood thinners or birth control, can also raise the prothrombin time.

In addition, factors such as malabsorption syndrome, which restricts the absorption of vitamin K in the intestinal tract, can distort the results of a prothrombin time test. Dehydration from severe vomiting or diarrhea can increase clotting times, notes WebMD. Consuming vitamin K supplements, herbal supplements or natural remedies can also distort the results. Other factors that affect results include consuming large amounts of alcohol or foods rich in vitamin K, such as kale, turnip greens, broccoli and soybeans.

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