High IgA antiphospholipid autoantibodies in healthy Sudanese explain the increased prevalence among Sudanese compared to Swedish systemic lupus erythematosus patients

OBJECTIVES: IgA antiphospholipid antibodies (aPL) are prevalent in systemic lupus erythematosus (SLE) patients of African American, Afro-Caribbean and South African origin. Nevertheless, data from North Africa are lacking, and most studies use manufacturer-suggested cut-offs based on Caucasian controls. Therefore, we compared aPL isotypes in Sudanese and Swedish SLE patients using nation-based cut-offs. METHODS: Consecutive SLE patients and age- and sex-matched controls from Sudan (N = 115/106) and Sweden (N = 340/318) were included. All patients fulfilled the 1982 American College of Rheumatology SLE classification criteria. Antiphospholipid syndrome–related events were obtained from patients’ records. IgA/G/M anticardiolipin and anti-β(2) glycoprotein I (β(2)GPI) were analysed with two independent assays. IgA anti-β(2)GPI domain 1 (D1) was also investigated. Manufacturers’ cut-offs and the 95th and 99th percentile cut-offs based on national controls were used. RESULTS: Sudanese patients and controls had higher levels and were more often positive for IgA aPL than Swedes when using manufacturers’ cut-offs. In contrast, using national cut-offs, the increase in IgA aPL among Sudanese patients was lost. Occurrence of IgA anti-D1 did not differ between the countries. Venous thromboses were less common among Sudanese patients and did not associate with aPL. No clinical associations were observed with IgA anti-β(2)GPI in Sudanese patients. Thromboses in Swedes were associated with IgG/M aPL. Fetal loss was associated with aPL in both cohorts. CONCLUSIONS: IgA anti-β(2)GPI prevalence was higher among Sudanese compared to Swedish patients when manufacturers’ cut-offs were used. This situation was reversed when applying national cut-offs. Anti-D1 was not increased in Sudanese patients. Previous studies on populations of African origin, which demonstrate a high prevalence of IgA aPL positivity, should be re-evaluated using a similar cut-off approach.