Lupus anticoagulant assay cut‐offs vary between reagents even when derived from a common set of normal donor plasmas

Abstract

BackgroundMulticenter studies reveal that diagnostic efficacy of lupus anticoagulant (LA) assays is enhanced if cut‐offs are locally generated. However, a potential confounder is the inevitable use of separate normal donor populations. ObjectivesGenerate cut‐offs for multiple LA reagents with the same analyzer and normal donor plasmas. MethodsCut‐offs for screen ratio, confirm ratio, percent correction of screen ratio by confirm ratio, and normalized screen/confirm ratio (NSCR) were derived from the same 50 normal donor plasmas for screen and confirm pairs for two dilute Russell's viper venom time reagents, LA1/LA2 and HEMOCLOT™ LA‐S/LA‐C, and two APTTs, Actin FSL/FS and Cephen LS/Cephen. The cut‐offs were challenged with plasmas from 20 triple‐positive APS patients and 25 plasmas from LA‐negative, thrombotic patients. ResultsCut‐offs for screen ratio, confirm ratio, percent correction, and NSCR, respectively, were 1.12/1.08/8.3/1.09 for LA1/LA2; 1.17/1.10/13.6/1.13 for HEMOCLOT™ LA‐S/LA‐C; 1.12/1.13/9.7/1.10 for Actin FSL/FS; 1.09/1.13/11.0/1.11 for Cephen LS/Cephen. LA1 and LA‐S screens were elevated in 19/20 and 16/20 triple‐positive plasmas, respectively, while 20/20 were detected with both via integrated interpretation ie, percent correction or NSCR irrespective of screen elevation. Actin FSL and Cephen LS screens were elevated in 17/20 and 19/20 triple‐positive plasmas, respectively, while one more LA was detected with Actin FSL via integrated interpretation, but not for Cephen LS. Integrated interpretation suggested 5/25 LA‐negative plasmas contained weak LA (two with Actin FSL/FS, two with LA1/LA2, one with LA‐S/LA‐R). ConclusionsEmploying the same normal donor plasmas and analytical platform does not compensate for between‐reagent differences when generating LA assay cut‐offs.