Depending on the reagent used for testing, test results are within normal limits despite the presence of clinically significant plasma concentrations of the NOAC drugs. In contrast, there may be occasions when test results are beyond the upper limit of the normal reference range, but the drug concentration is relatively low.
The presence of heparin may erroneously mimic the effect of anti-Xa drugs. Some commercial anti-Xa assays for NOAC have added heparin neutralizers to overcome this interference. The laboratory should check with the manufacturer for the presence of such neutralizers in the method used locally.
Drug interference for NOAC may have unknown effects for certain common drugs. Laboratory testing before and a few days after initiating new drugs may provide important information.
Coagulation-related abnormalities can be due to hemostatic conditions, a false positive result with lupus anticoagulant, or an activated protein C assay with an abnormal result. To block interference from NOACs, agents have been developed to remove or block NOACs in the test tube to correctly diagnose the various hemostatic conditions and confirm or exclude a diagnosis of lupus anticoagulant and inherited thrombophilias. Activated charcoal or carbon in products such as NOAC Stop was found to remove all types of NOACs, including dabigatran, apixaban, rivaroxaban, and edoxaban, from test plasmas with minimal effect on any of the main clotting tests.
Alternate testing methodologies such as chromogenic assays not involving Xa or IIa should be considered for use. Aside from the issues with monitoring NOACs, it is important to know that NOACs can interfere with specialized coagulation testing and that this testing should generally be avoided when NOACs are present.
