IntroductionTesting for lupus anticoagulant (LA) is not recommended in case of inflammation, as C-reactive protein (CRP) can interfere in vitro with the phospholipids present in the activated partial thromboplastin test (aPTT) used to detect a LA. However, the potential interference of an acute phase protein (i.e. CRP) on LA testing using the dilute Russell's viper venom (DRVV) test is poorly studied. MethodsFirst, a retrospective analysis (2013-2023) was performed: all LA work-ups were retrieved and the association between CRP levels and DRVV screen, mix and confirm clotting times was studied. Second, DRVV panels and CRP levels were extracted from two prospective studies involving intensive care unit patients to study the association between both variables. Third, CRP was added to normal pool plasma at six relevant concentrations (up to 416 mg/L) to study the association between CRP itself and DRVV coagulation times. ResultsIn the retrospective analysis, DRVV screen and confirm clotting times significantly increased as CRP rose (increase of 0.11 s and 0.03 s per 1 mg/L increase of CRP level, respectively). In the prospective analysis, only DRVV screen was prolonged with high CRP levels (increase of 0.06 s for a 1 mg/L increase in CRP level); DRVV screen/confirm ratio was also increased with high CRP levels. In vitro, the addition of CRP did not significantly increase any DRVV clotting times. ConclusionLA testing should be performed with much caution in the presence of inflammation, as it may be associated with prolongation of both aPTT and DRVV clotting times.