Clinical and laboratory characteristics of patients with false positive IgM test for SARS-CoV-2

The widespread occurrence of COVID-19 places significant pressure on diagnostic laboratories, potentially compromising the accuracy of tests performed and leading to an increase in false-positive results. The aim was to assess the incidence of false-positive IgM results to SARS-CoV-2 and to identify the characteristics of patients with such results. Materials and Methods. 102 participants were included in this retrospective study: 1) medical staff with double positive IgM and negative IgG results to SARS-CoV-2; 2) staff with double negative IgM and IgG results; 3) control group – patients with coronavirus infection who were hospitalised for treatment. Laboratory examination included determination of autoimmune disease markers and antibodies to SARS-CoV-2 using different methods (ICA, IHLA, ELISA), and determination of virus-neutralising antibodies. Results. Groups 1 and 2 showed cases of chronic autoimmune thyroiditis (8%), psoriasis (4%), non-specific ulcerative colitis (1.5%) and antiphospholipid syndrome (1.5%). There were no statistically significant differences in the incidence of chronic diseases or markers of autoimmune diseases between study groups 1 and 2. In the first group, 13% of participants were negative for SARS-CoV-2 IgM when ICLA was used in the second blood sample; positive or doubtful results with other test systems ranged from 29% to 59% of observations. IgG seroconversion as determined by ICLA was not detected in any of the tests in the first group. Thus, the most convincing evidence of false-positive antibody to SARS-CoV-2 is the absence of seroconversion by IgG detected in the paired serum study with one system. To monitor the timing of seroconversion, COVID-19 patients (group 3) were tested: sera from all patients contained IgG to SARS-CoV-2 on days 17-19 of illness. Conclusion. No statistically significant effect of concomitant autoimmune pathology on false-positive IgM results to SARS-CoV-2 was found. However, the likelihood of repeated false-positive results was determined when using test systems with similar antigens.