BACKGROUND: During COVID‐19 pandemic, the early diagnosis of patients is a priority. Serological assays, in particular IgM and IgG anti‐SARS‐CoV‐2, have today several applications but the interpretation of their results remain an open challenge. Given the emerging role of the IgA isotype in the COVID‐19 diagnostics, we aimed to identify the SARS‐CoV‐2 IgA antibodies in a COVID‐19 population seronegative for IgM. METHODS: A total of 30 patients hospitalized in San Giovanni di Dio Hospital (Florence, Italy) for COVID‐19, seronegative for IgM antibodies, have been studied for anti‐SARS‐CoV‐2 antibodies. They all had a positive oro/nasopharyngeal swab reverse transcription polymerase chain reaction result. Assays used were a chemiluminescent assay measuring SARS‐CoV‐2 specific IgM and IgG (S+N) and an ELISA, measuring specific IgG (S1) and IgA antibodies against SARS‐CoV‐2. RESULTS: Among the 30 patients, eight were positive for IgA, seven were positive for IgG (N+S) and two for IgG (S1), at first point (5‐7 days from the onset of symptoms). The IgA antibodies mean values at the second (9‐13 days) and third (21‐25 days) time points were even more than twice as high as IgG assays. The agreement between the two IgG assays was moderate (Cohen's K = 0.59; SE = 0.13). CONCLUSIONS: The inclusion of the IgA antibodies determination among serological tests of the COVID‐19 diagnostic is recommended. IgA antibodies may help to close the serological gap of the COVID‐19. Variations among anti‐SARS‐CoV‐2 IgG assays should be considered in the interpretation of results. This article is protected by copyright. All rights reserved.
Closing the Serological Gap in the Diagnostic Testing for COVID‐19: The Value of Anti‐SARS‐CoV‐2 IgA Antibodies