Quantitative measurement of anti-interferon-gamma autoantibodies for routine laboratory implementation.

Anti-interferon-γ autoantibodies (AIGAs) are the immunological hallmark of adult-onset immunodeficiency (AOID) syndrome, predominantly affecting southeast Asian adults and leading to severe, recurrent opportunistic infections. Current AIGA detection relies on semi-quantitative methods that lack standardization, limiting utility for clinical monitoring and multi-center comparisons. We developed and analytically validated a quantitative inhibitory ELISA reporting half-maximal inhibitory concentration (IC50). The assay demonstrated excellent linearity across log10 AIGA concentrations of 2.51-4.92 (corresponding to 337-85,316 RU/mL). The quantifiable range could be extended down to 93 RU/mL (relative to the internal quality-control [IQC] plasma pools). The inter-assay GCV% was 23.5%-30.7% across all IQC AIGA-positive levels. Levey-Jennings monitoring confirmed stable performance of a single set of reagents throughout the 8-month clinical service period. The quantitative IC50 assay discriminated active cases from inactive with 82.86% sensitivity with 89.29% specificity versus a semi-quantitative reference method (72.86% sensitivity and 85.71% specificity). This quantitative ELISA provides reliable, precise AIGA measurement with a practical internal quality-control framework, supporting AIGA testing for routine clinical laboratory implementation in AOID management.