The Challenge Of Russell’s Viper Venom Time Interpretation For Lupus Anticoagulant Diagnosis: A Two-Year Retrospective Institutional Review

The UCSF hematology laboratory uses the Precision Biologic LA Check and LA Sure reagents for the Russell’s viper venom time (RVVT), as part of the laboratory diagnosis of a lupus anticoagulant (LA). Currently, manufacturer’s FDA-approved guidelines for interpretation differ from the recommended diagnostic criteria of the International Society for Thrombosis and Hemostasis (Pengo V et al. JTH. 2009, 7:1737). The manufacturer’s guideline for positivity is patient results 1) >2 SD above the mean of normal donors for the screening test and 2), >2 SD above the mean of the ratio of the LA Check time to phospholipid-supplemented LA Sure time in normal donors. The ISTH guideline for positivity is patient results 1) >99^th percentile of normal donors for the screening test, 2) >99^th percentile of the ratio of mix of patient and pooled normal plasma (PNP) to the mix of normal donors and PNP with the screening reagent, and 3) >50^thpercentile of the mean of the ratio of the LA Check time to phospholipid-supplemented LA Sure time in normal donors. Therefore, some cases meet one or the other definition of LA, but not both.

We interpret the RVVT as positive when both manufacturer’s and ISTH guidelines are met, negative when neither criteria are met, and equivocal when one set of criteria is met but not the other. Some authors have advocated using a similar “third” category of “indeterminate lupus anticoagulant” in order to highlight the potentially elevated risk for these patients to have pro-thrombotic events (Park SH et al. Korean J Hematol. 2012, 47:83).

Here we report our two-year experience (7/6/2011-7/6/2013) with patients with prolonged screening RVVT (using LA Check). Those samples with prolonged screening RVVT greater than 2 standard deviations above the average of normal donors undergo additional testing: a 1:1 mix with PNP (using LA Check) and high phospholipid confirmatory testing (using LA Sure). We retrospectively reviewed all samples with a prolonged screening RVVT in a two year period to determine the frequency of deviations between the manufacturer’s and ISTH guidelines, as well as the frequency of prolonged screening RVVT. We also correlate the RVVT results with the other lupus anticoagulant assay used at UCSF, the Staclot® LA 20 (utilizing hexagonal phase phosphatidylethanolamine)

329 samples had a prolonged screening RVVT, representing 321 patients (24 patients had two RVVT performed, 2 patients had three RVVT). 1477 total RVVT were performed in the study period (i.e. 22.3% had a prolonged screen). Of the 329 samples with a prolonged screening RVVT, 116 were positive (35.3%), 139 were negative (42.2%), and 74 were equivocal (22.5%). Of the 74 cases with an equivocal RVVT, 36 (48.6%) had a positive result according to manufacturer’s guidelines and 38 (51.4%) had a positive result according to ISTH guidelines

Of the 329 samples with a prolonged screening RVVT, 122 had a concurrent negative Staclot® LA 20 assay (37%), 16 had a concurrent equivocal Staclot® LA 20 assay (4.9%), 172 had a concurrent positive Staclot® LA 20 assay (52.3%), and 19 did not have a concurrent Staclot® LA 20 assay (5.8%).

Of the 74 cases with an equivocal RVVT, 71 had a concurrent Staclot® LA 20 assay. Of those 71, 25 (35.2%) had a negative Staclot® LA 20, and 4 (5.6%) had an equivocal Staclot® LA 20, and 42 (59.2%) had a positive Staclot® LA 20. Of the 42 cases with an equivocal RVVT and a concurrent positive Staclot® LA 20 assay, 13 (31%) met RVVT criteria for manufacturer’s guidelines and 29 (69%) met RVVT criteria for ISTH guidelines.

Of the 329 samples with a prolonged screening RVVT, 47 were associated with identified concurrent anticoagulant treatment. 17 (36.2%) of these 47 had a negative RVVT, 13 (27.7%) had an equivocal RVVT, and 17 had a positive RVVT (36.2%).

Overall, this study has shown that there are frequent cases (22.5%) where the RVVT assay meets one or the other guidelines for a positive LA. The equivocal diagnoses were approximately evenly split between those cases positive by manufacturer’s guidelines or positive by ISTH guidelines. Compared to the overall sample, those cases with an equivocal RVVT did not more frequently have a positive Staclot® LA 20 assay. Those cases with an equivocal RVVT who were positive by ISTH guidelines did seem to enrich for a positive Staclot® LA 20 assay, potentially reflecting a better sensitivity of the ISTH guidelines for those patients with an LA as compared to the manufacturer's guidelines.