If an anti-FXa assay is unavailable, the prothrombin time (PT) may be useful for qualitative assessment. A prolonged PT suggests the presence of a NOAC therapeutic drug at or above therapeutic levels for these agents. PT can be prolonged or demonstrate no effect on the test results with any of the NOAC that target factor Xa or thrombin, but also by other defects. Whenever the PT is used for patients on NOAC, results should be expressed as the ratio of patient-to-normal clotting time. However, depending on the sensitivity of the PT reagent, a normal PT does not exclude the presence of a therapeutic drug, nor does it exclude patients who have above therapeutic levels of the drug.
When interpreting PT results in patients on NOAC, it should be considered that the test might be prolonged because of defects of coagulation factors rather than the drug taken by the patient.
