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

Pylaeva S.K., Sayfullin R.F., Karan L.S., Stukolova O.A., Kozlovskay L.I., Sinyavkin D.O., Ishmukhametov A.A.

Chumakov Federal Scientific Center for Research and Development of Immune-and-Biological Drugs of the Russian Academy of Sciences, Moscow, Russia
Pirogov Russian National Research Medical University of the Ministry of Health of the Russian Federation, Moscow, Russia
Central Institute of Epidemiology, Moscow, Russia
Sechenov First Moscow State Medical University, Russia
City Clinical Hospital No. 52 of the Department of Health of the City of Moscow, Russia

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.