How to report results of prothrombin time and activated partial thromboplastin time

Prothrombin time (PT) and activated partial thromboplastin time (APTT) are the most widely used tests to investigate coagulation abnormalities. Varied result reporting have been introduced over the years for the two tests, thus making their interpretation rather confusing in different clinical settings. PT results have been reported as clotting time, percentage activity, PT-ratio (patient-to-normal clotting time) and as international normalized ratio (INR). The INR scale has been devised to harmonize results stemming from different thromboplastins from patients on treatment with vitamin K antagonists. Therefore, there are some theoretical and evidence-based considerations that make the INR formally invalid when the test is used to analyze patients in other clinical settings. Unfortunately, this limitation has been frequently overlooked, and the INR has been (and is currently) used as a universal system of results harmonization. The APTT has been historically reported as clotting time or as ratio (patient-to-normal clotting time). In this opinion paper we review the current state-of-the-art for result reporting and attempt to give practical guidance on how PT and APTT should be reported in different clinical conditions for which the tests are requested.