False-positive detection of IgM anti-severe acute respiratory syndrome coronavirus 2 antibodies in patients with rheumatoid arthritis: Possible effects of IgM or IgG rheumatoid factors on immunochromatographic assay results

Objectives: The severe acute respiratory syndrome coronavirus 2 causes coronavirus disease 2019. A serological test is conducted to determine prior infection by severe acute respiratory syndrome coronavirus 2. We investigated whether the results of anti-severe acute respiratory syndrome coronavirus 2 antibody tests are modified in patients with rheumatoid arthritis. Methods: Patients in Japan with rheumatoid arthritis were recruited at Sagamihara Hospital from July 2014 to October 2015 (n=38; 2014 cohort) and at Tokyo Hospital from June to October 2020 (n=93; 2020 cohort). Anti-severe acute respiratory syndrome coronavirus 2 antibodies were measured by electrochemiluminescence immunoassay or immunochromatographic assay. Results: Anti-severe acute respiratory syndrome coronavirus 2 antibodies were not detected in any of the samples from rheumatoid arthritis patients tested by electrochemiluminescence immunoassay. Anti-severe acute respiratory syndrome coronavirus 2 antibodies were detected by immunochromatographic assay in the 3 (7.9%) serum samples in the 2014 cohort and 15 (16.1%) serum samples in the 2020 cohort. The IgM rheumatoid factor levels were increased in rheumatoid arthritis patients with IgM anti-severe acute respiratory syndrome coronavirus 2 antibodies detected by immunochromatographic assay (mean +/- standard deviation (IU/ml), 1223.0 +/- 1308.7 versus 503.6 +/- 1947.2; P=0.0101). The levels of IgG rheumatoid factor were also upregulated in rheumatoid arthritis patients with IgM anti-severe acute respiratory syndrome coronavirus 2 antibodies detected by immunochromatographic assay (4.0 +/- 0.7 versus 2.4 +/- 0.9; P=0.0013). Conclusion: The results of IgM anti-severe acute respiratory syndrome coronavirus 2 antibody testing by immunochromatographic assay are modified by IgM or IgG rheumatoid factors in rheumatoid arthritis patients.