The association of serum antiphospholipid antibodies and dilute Russell's viper venom times.

Aims: We hypothesised that there is a threshold value for the association of dilute Russell's viper venom times (dRVVT) with positive immunoglobin G antiphospholipid antibody (IgG-APLA) test results.

Methods: We tested 120 controls and a cohort of 2412 outpatients who had concomitant test results for dRVVT and IgG-APLA (IgG antibodies to cardiolipins and β2-glycoprotein I). We also selected a subgroup who had repeated IgG-APLA tests at least 12 weeks apart (1398 patients with multiple β2-glycoprotein I tests and 672 with multiple aCL tests). We cross tabulated the proportion of IgG-APLA single positive, double positive and persistently positive antibodies with dRVVT values.

Results: The distribution of the dRVVT results from the reference population was consistent with an upper limit of the reference interval of 1.22 to >1.48. A consistent increase in the proportion of IgG-APLA single, double positive and persistently positive antibody tests occurred in the group with a normalised dRVVT ratio of 1.40-1.49. IgG-APLA double positivity was found in 12.5% (4 of 32) patients with a ratio of dRVVT 1.40-1.49 compared with 3.3% (6/181) of those with a ratio of dRVVT 1.20-1.39 (p=0.045).

Conclusions: We conclude that there is an association between dRVVT positivity and elevated proportions of single, double and persistently positive IgG-APLA test results with an apparent threshold effect. These findings may provide a general guide to risk and suggest a way to choose from a wide range of possible upper limits of the reference interval.