Testing for the Lupus Anticoagulant (LA): the good, the bad, and the ugly.

Lupus anticoagulant (LA) represents one of the laboratory criteria for classification of patients as definite antiphospholipid syndrome (APS). The other two laboratory criteria are anticardiolipin antibodies (aCL) and anti-beta2-glycoprotein I antibodies (aβ2GPI). At least one of these antiphospholipid antibody (aPL) tests need to be positive, with evidence of persistence, together with evidence of at least one clinical criterion for APS, before a patient can be classified as having definite APS. LA and other aPL assays are also important for diagnosis or exclusion of APS, as well as for risk stratification, with triple positive patients carrying the greatest risk. Whereas LA is identified through ‘uncalibrated’ clot-based assays, the other aPL assays (aCL, aβ2GPI) represent immunological assays, identified using calibrated solid phase methods. Because LA are identified using clot-based assays, they are subject to considerable pre-analytical and analytical issues that challenge accurate detection or exclusion of LA. In this narrative review, we take a look at the good, the bad, and the ugly of LA testing, primarily focusing on the last 10 years. Although harmonization of LA testing as a result of ISTH guidance documents and other international activities has led to improvements in LA detection, many challenges remain. In particular, several anticoagulants, especially direct oral anticoagulants (DOACs), and also vitamin K antagonists, given as therapy to treat the pathophysiological consequences of aPL, especially thrombosis, interfere with LA assays, and can generate false positive or false negative LA findings. Overcoming these diagnostic errors will require a multifaceted approach with clinicians and laboratories working together.