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.