If there is more than a 10% difference in the clotting times between duplicate samples, the tests on that plasma sample should be repeated. Tests are historically performed on 20 normal donors not on anticoagulants and 60 patients who have been on oral anticoagulant treatment for at least 6 weeks. Prothrombin Times are performed in duplicate for each thromboplastin and the mean for each pair of tests derived. human against human, rabbit against rabbit etc. The calibration of a test thromboplastin must be against a reference thromboplastin of the same species e.g. Subsequent WHO IRPs contain no adsorbed bovine plasma. The first WHO IRP was a human brain extract to which adsorbed bovine plasma was added to optimise the content of the non-vitamin K dependent coagulation factors. The first WHO reference thromboplastin was assigned an ISI of 1.0 and it is against this (and subsequent reference preparations) that all other thromboplastins are calibrated. Individual thromboplastins can be calibrated against an international WHO reference thromboplastin (International Reference Preparation or IRP) to assign them an International sensitivity index or ISI. This difference in sensitivities is known as the sensitivity index. rabbit, human, bovine etc) used in the assay. The Prothrombin Time (PT) in an individual with one or more deficiencies of a clotting factor will vary with the type of thromboplastin (e.g. International Sensitivity Index & The International Normalised Ratio A longer-than-normal PT can be caused by treatment with blood-thinning medicines, such as warfarin or, in rare cases, heparin.This is a life-threatening condition in which your body uses up its clotting factors so quickly that the blood cannot clot and bleeding does not stop. A longer-than-normal PT can also mean that you have disseminated intravascular coagulation (DIC). It can also mean a lack of vitamin K liver disease, such as cirrhosis or that a liver injury has occurred. A longer-than-normal PT can mean a lack of or low level of one or more blood clotting factors (factors I, II, V, VII, or X).Prothrombin times are also kept at longer times for people with artificial heart valves, because these valves have a high chance of causing clots to form. The warfarin dose is changed so that the prothrombin time is longer than normal (by about 1.5 to 2.5 times the normal value or INR values 2 to 3). In some labs, only the INR is reported and the PT is not reported. Using the INR system, treatment with warfarin (Coumadin) will be the same. A value that isn't in the normal range may still be normal for you.Ī method of standardizing prothrombin time results, called the international normalized ratio (INR) system, has been developed so the results among labs using different test methods can be understood in the same way. Your doctor will also look at your results based on your age, health, and other factors. Your lab report should show the range that your lab uses for each test. An increase in the use of the clotting factors.Īn abnormal prothrombin time is often caused by liver disease or injury or by treatment with blood thinners.Įach lab has a different range for what's normal.Other substances, called inhibitors, that affect the clotting factors.The absence of any of the clotting factors.A change in the activity of any of the clotting factors.Blood-thinning medicine, such as warfarin.

Prothrombin time is an important test because it checks to see if five different blood clotting factors (factors I, II, V, VII, and X) are present. Vitamin K is needed to make prothrombin and other clotting factors. Prothrombin, or factor II, is one of the clotting factors made by the liver. In some labs, only the INR is reported and the PT is not reported.īlood clotting factors are needed for blood to clot (coagulation). It lets your doctor understand results in the same way even when they come from different labs and different test methods. INR (international normalized ratio) stands for a way of standardizing the results of prothrombin time tests, no matter the testing method. PT is also used to check whether medicine to prevent blood clots is working.Ī PT test may also be called an INR test. A prothrombin time test can be used to check for bleeding problems. Prothrombin time (PT) is a blood test that measures how long it takes blood to clot.

Inr normal range 4