Optimal ImmunoCAP Aspergillus fumigatus‐Specific IgE Threshold for Detecting Allergic Bronchopulmonary Aspergillosis in Adults With Asthma: A Diagnostic Accuracy Study

Background Allergic bronchopulmonary aspergillosis (ABPA) requires screening in adults with asthma using Aspergillus fumigatus ‐specific IgE (Af‐IgE). The current Af‐IgE threshold of 0.35 kUA/L has low specificity, leading to unnecessary downstream testing. Objectives To determine an optimal Af‐IgE threshold that maintains high sensitivity while improving specificity for ABPA screening in adults with asthma. Methods We performed a diagnostic accuracy study of prospectively collected data using distinct derivation and validation cohorts in a tertiary care setting. The index test was ImmunoCAP Af‐IgE at thresholds of 0.35, 0.70 and 0.90 kUA/L. The reference standard was the 2024 ISHAM‐ABPA criteria applied by investigators aware of all test results. Results We included 543 consecutive asthmatic subjects investigated for possible ABPA in the derivation cohort (July 2017–September 2018) and 375 subjects with established asthma and ABPA in the validation dataset (January 2020–December 2023). ABPA prevalence was 19.5% (106/543) in derivation and 69.6% (261/375) in validation cohorts. Bayesian latent class analysis estimated a 0.3 kUA/L cut‐off but did not improve specificity; frequentist analysis showed model misfit. In the validation cohort, increasing the threshold from 0.35 to 0.70 kUA/L improved specificity from 66.7% (95% CI, 57.6–74.7) to 72.8% (95% CI, 64.0–80.1; p = 0.016) without significant sensitivity reduction (100% to 98.9%; 95% CI, 96.7–99.6; p = 0.25). No patients with bronchiectasis were missed. At 0.90 kUA/L, sensitivity significantly decreased (97.7%; 95% CI, 95.1–98.9; p = 0.031). Conclusions ImmunoCAP Af‐IgE threshold of 0.70 kUA/L optimised diagnostic performance for ABPA screening in adults with asthma, potentially reducing unnecessary confirmatory testing.