Prothrombin time (PT) is a clotting test that preferentially measures the function of the extrinsic clotting pathway. As laboratory reagents can vary from hospital to hospital, the values of the prothrombin time can also vary (normal range between 9 and 12 seconds). To compensate for these variations, adjustments are made for to give a standardised index called the International Normalised Ratio INR. This is used in preference to PT to ensure uniformity across laboratories.
The main use for the INR is to control anticoagulation treatment with warfarin, it is also used as an index of liver disease including poisoning with paracetamol. In chronic liver disease, the prothrombin time may not accurately reflect actual clotting function as production of both the pro-thrombotic and anti-thrombotic clotting factors may be impaired in approximately equal measure. For this reason, paracentesis in patients with cirrhosis may be carried out despite apparently deranged clotting.