Determination of Positivity Cutoff for an Automated Aspergillus fumigatus-Specific Immunoglobulin-G Assay in a National Reference Laboratory.

Background: Detection of serum-specific immunoglobulin G (sIgG) to Aspergillus fumigatus traditionally relied on precipitin assays, which lack standardization and have poor analytical sensitivity. Automated quantitative immunoassays are now more widely used alternatives. A challenge, however, is determining reference interval (RI) cutoffs indicative of disease presence.

Methods: Sera from 152 local healthy donors were tested for Aspergillus fumigatus sIgG using the ImmunoCAP assay to calculate a nonparametric RI cutoff. Results from 178 patient samples cotested by the precipitin and ImmunoCAP assays were analyzed using receiver operator characteristic (ROC) curve to determine an optimal sIgG concentration for precipitin positivity. Clinical information available for 46 patients tested by the ImmunoCAP assay was also used to estimate an optimal sIgG cutoff for pulmonary aspergillosis diagnosis.

Results: Specific-IgG concentration at 81.5 mcg/mL corresponded to the 97.5th percentile of tested healthy donors. The ROC-driven optimal IgG cutoff for precipitin positivity was at 40.4 mcg/mL with 67.8% sensitivity [95% confidence interval (CI): 54.4% to 79.4%%] and 72.3% specificity (95% CI: 63.3% to 80.1%). Using clinical diagnoses, an IgG concentration at 64.7 mcg/mL had optimal sensitivity (77.8%; 95% CI: 61.9% to 88.3%) and specificity (66.7%, 95% CI 39.1% to 86.2%) for pulmonary aspergillosis.

Conclusions: Our healthy donor-driven RI cutoff was higher than estimated optimal sIgG values based on precipitin positivity and disease presence. As fungal sIgG levels can be impacted by local environmental exposures, and given the limited size of our clinical dataset, adopting an assay cutoff based on precipitin results (40.4 mcg/mL) can be more objective.