Results expression for tests used to measure the anticoagulant effect of new oral anticoagulants

Results of clotting tests used to measure the effect of old and new antithrombotic drugs can be expressed in different ways and this is considered as one of the sources of variability to explain the differences of results obtained for the same patient plasma when tested in different laboratories. This is particularly important for patient on vitamin K antagonists and led to the development of the international normalized ratio system of results reporting in this setting. Although standardization of results expression for the tests meant to measure the anticoagulant effect of new oral anticoagulants (NOA) is presently not perceived as an issue, it may become crucially important at the time when test-specific cut off values will be available to help assessing the risk of bleeding in individual patients who are on over-dosage. Effort should therefore be made to harmonize as much as possible results obtained in different laboratories using the same method, but different reagents. This article is aimed at discussing different options of results reporting of tests for NOA and their merits/pitfalls.