Prothrombin time

The prothrombin time (PT) test, standardised as the INR test is most often used to check how well anti-coagulant or ‘blood-thinning’ tablets such as warfarin and phenindione are working. Anti-coagulant tablets help prevent the formation of blood clots (they do not ‘thin the blood’ as is popularly thought). This is particularly important in people with heart conditions such as atrial fibrillation or artificial valves, or people with a history of recurrent blood clots. The drug‘s effectiveness can be determined by how much it prolongs the PT (measured in seconds), or increases the INR (a standardised ratio of the patient‘s PT versus a normal sample).

If you are taking an anti-coagulant drug, your doctor will check your INR regularly to make sure that your prescription is working properly and that the INR is appropriately increased. There is no set frequency for doing the test. Your doctor will order it often enough to make sure that the drug dose is right. Occasionally the PT or INR may be used on a patient who is not taking anti-coagulant drugs — to check for a bleeding disorder, liver disease or vitamin K deficiency, or to ensure clotting ability before surgery.

The test result for PT depends on the method used; results will be measured in seconds.

Most laboratories report PT results that have been adjusted to the International Normalised Ratio (INR). Patients on anti-coagulant drugs usually have a target INR of 2.0 to 3.0 (i.e. a prothrombin time 2 to 3 times as long as in a normal patient, using standardised conditions). For some patients who have a high risk of clot formation, the INR needs to be higher: about 3.0 to 4.0. Your doctor will use the INR to adjust your drug to get the PT into the range that is right for you. An increased prothrombin time or INR means that your blood is taking longer to form a clot. If you are not taking anti-coagulant drugs and your PT is prolonged, additional testing may be necessary to determine the cause. Result of the PT is often interpreted with that of the aPTT in determining what condition may be present.

Interpretation of PT and aPTT in patients with a bleeding syndrome

Some substances you consume — such as alcohol and different drugs, can interfere with the PT test and give a misleading result. Antibiotics, aspirin and cimetidine can increase PT. Barbiturates, oral contraceptives and hormone-replacement therapy (HRT), and vitamin K - either in a multivitamin or liquid nutrition supplement - can decrease PT. Make sure that your doctor knows all the drugs you are taking so that the PT results are interpreted correctly.