Diagnostic performance of IgG against multiplex <i>Aspergillus</i> antigens (mx4) for identifying chronic pulmonary aspergillosis

Abstract Serum A. fumigatus-IgG is the cornerstone serological test for diagnosing chronic pulmonary aspergillosis (CPA), but its sensitivity is imperfect. A multiplex Aspergillus antigen preparation (mx4) incorporating extracts of A. fumigatus, A. flavus, A. niger, and A. terreus is commercially available. We aimed to compare the diagnostic performance of the serum mx4-IgG assay with the A. fumigatus-IgG assay for diagnosing CPA. We prospectively enrolled 332 consecutive adults with suspected CPA at a tertiary chest clinic between August 2022 and July 2025. Serum IgG against mx4, A. fumigatus, A. flavus, A. niger, and A. terreus was measured using a fluorescent enzyme immunoassay. Diagnostic performance was evaluated against a primary composite reference standard and two alternative serology-based definitions. Hierarchical diagnostic algorithms and a simplified cost analysis were performed. Of 332 participants, 230 had CPA, and 102 were diseased controls (patients with structural lung disease). Against the primary reference standard, the mx4-IgG assay achieved a sensitivity of 83.0% and specificity of 73.5%, compared with 95.2% and 88.2%, respectively, for the A. fumigatus-IgG assay. Similar findings were observed using alternative serology-based CPA definitions. In hierarchical testing, a sequential strategy of A. fumigatus-IgG and A. flavus-IgG identified 97.7% of CPA cases at the lowest cost (USD 24/patient) and outperformed strategies incorporating the mx4-IgG assay (mx4-IgG followed by A. fumigatus-IgG). The mx4-IgG assay was not superior to the A. fumigatus-IgG assay for diagnosing CPA. Sequential testing with A. fumigatus-IgG followed by A. flavus-IgG achieved the highest diagnostic yield at the lowest cost and warrants further evaluation.