The laboratory diagnosis of lupus anticoagulant in patients on oral anticoagulation.

Laboratory-specific cut-off lupus ratios (LR), above which a plasma is judged positive for lupus anticoagulant (LA), were established for both activated partial thromboplastin time-based and dilute Russell viper venom time-based methods. The validity of using these cut-off values to determine the presence of LA in patients on oral anticoagulation (OAC) was assessed. A cohort of 40 patients (23 male and 17 female), aged 22-84 years (mean 52 years) were tested for LA at the time of a thrombotic event. Repeated testing was performed after the same patients were treated with OAC (international normalized ratio 2.0-3.5). For 36 patients (90%), LA status was unchanged pre- and on-OAC. Thirteen of the 40 patients (32.5%) were positive for LA both pre- and on-OAC. Of the 27 patients negative for LA pre-OAC, 23 remained negative on-OAC. The four discordant results were interesting in that LA positivity was demonstrated only after the patient was stable on-OAC. In our cohort of 40 patients, there was a trend for LRs to decrease on-OAC, but this did not reach statistical significance. The subset (4) went against this trend and became positive after the thrombotic event.