Fig. 2.

Factors which interfere with the interferon gamma release assay (IGRA) and the enzyme linked immunosorbent assay (ELISA). A: Common immunoassay interfering factors to be considered in SLE: lymphocyte count, phytohemagglutinin (PHA) capacity to induce T cell memory and NK responses, gamma-globulinemia level, albuminemia level, and C-reactive protein (CRP) level. B: Comparison of these factors according to the IGRA-Spike and IgG anti-Spike antibody status in SLE patients. For statistical analysis a T-test was used and results are expressed as –log10 of the p value. C: Receiving operating characteristic (ROC) curve to establish at which serum albumin level can discriminate using Youden's index IGRA-S positive from IGRA-S negative SLE patients. D: IGRA-Spike results in SLE patients according to the serum albumin threshold fix at 40 g/L. P values and assay thresholds (dot line) are indicated.