Medicines

DOACs could muddy diagnostic waters


Doctors could risk wrongly diagnosing certain conditions in patients who are taking direct acting oral anticoagulants (DOACs) say experts who warn that the blood thinners can affect a variety of pathology test results.

Speaking to the limbic, Dr Emmanuel Favalaro from the Institute of Clinical Pathology and Medical Research at Westmead Hospital in Sydney said many doctors are not aware that DOACs could strongly affect the results of certain tests they may order to make a diagnosis related to a thrombotic or bleeding event.

“Most DOACs affect routine coagulation tests such as prothrombin time (PT) and/or activated partial thromboplastin time (APTT), with dabigatran affecting the APTT more than the PT, and rivaroxaban affecting the PT more than the APTT.

All DOACs affect more complex assays such as factor assays, or those assays used to diagnose lupus anticoagulant, which is a risk factor for thrombosis,“ he told the limbic.

The effect of DOACs on these particular assays is widely underappreciated says Dr Favalaro because it has not been a concern in the past with older antithrombotic therapies like warfarin and heparin.

While these anticoagulants do interfere with clotting assays, there are a number of strategies that drug manufacturers have developed to neutralise their affects.

“The manufacturers of heparin for example are so much aware of its affects on thrombophilia tests that they introduced a heparin neutraliser. And although vitamin K antagonists can also affect clotting assays we can mix assays with normal plasma, which neutralises the warfarin effect.”

But Dr Favalaro says that manufacturers have no such neutralising strategy for the DOACs other than to advise against performing clot-based assays while patients are on the medications.

“The DOACs act like specific inhibitors so if a doctors requests a thrombophilia screen for example and that patient is on a DOAC they will actually give a laboratory test pattern that is in some way similar to actual lupus anticoagulant and they will get a false positive diagnosis with lupus anticoagulant if that test is requested.”

The reverse is true for patients on DOACs who actually do have lupus anticoagulant, he added.

“Depending on the way you do the assay, the effect of a DOAC on these assays, may actually lead to the conclusion that lupus anticoagulant is not there even when it is.”

According to Dr Favalaro, whose hospital lab carries out about a hundred coagulant tests each week, a major problem is that laboratories are not told when patients are on DOACs.

If doctors aren’t aware that, for some patients, a test result may in fact be an artefact of their DOAC use, it could mean a wrong diagnosis.

“We have to work out [a patients DOAC status] somehow as part of the processes of the investigation so that we can properly interpret the test findings but that’s very difficult for us – as laboratory staff we can only do the tests that are requested of us … if we do additional tests that’s considered over servicing and we either can’t charge for those additional tests or we get criticised for doing them.”

But now Dr Favaloro is pushing for greater communication between the doctors who order such tests and the labs that perform them in a bid to improve outcomes for patients.

“An effort to establish clear communication with the wider medical community who manage patients on DOACs, including all complexities relating to these treatments, would be beneficial.”

He said expert guidelines do recommend that coagulant testing should not be undertaken when patients are on DOACs or to discuss those issues with the laboratory involved in carrying out the assay before ordering haemostasis tests in patients on DOACs.

“Some expert recommendations suggest patients be transitioned to other drugs, like low molecular weight heparin for example, that don’t interfere with the test but often clinicians don’t want to do that – and I can understand that – so the usual recommendation from us is that the patient gets properly investigated once they get off all medications that may affect these tests.”

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