Non-specific reactions in the determination of specific IgG antibodies of hypersensitivity pneumonitis within ImmunoCAP®: frequencies and clinical significance

Specific IgG antibodies against fungi, bacteria, and avian antigens are of great relevance in serologic diagnosis of hypersensitivity pneumonitis. With the determination of IgG antibodies, false-positive results can occur due to non-specific reactions. In the present study, this was observed in a collective of - 10,000 patient sera tested for specific IgGs using ImmunoCAP (R) between April 2017 and December 2018 in 2% (203 patients). Nonspecific background activity can be detected in this test system by an additional determination of IgG(s) against MBP (maltose-binding protein) or HSA (human serum albumin). By substracting the MBP readings from the IgG-CAP readings, the real specific IgG antibodies can be identified. With thermophilic bacteria and moulds, normalization of the results occurs with greater frequency after this correction than with avian antigens. 58% of the MBPpositive patients had a definite diagnosis of hypersensitivity pneumonitis, in only 19% hypersensitivity pneumonitis was excluded. When diagnosing hypersensitivity pneumonitis, it should always be remembered that in patients whose IgG readings are all above the cut-off limits there may be a non-specific component and that there is the possibility of determining this with an additional test using IgG-ImmunoCAP (R).