NEW YORK — April 17, 2020 — Researchers have identified factors that lead to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) IgM false-positives. The findings are published in Journal of Clinical Microbiology.
Qiang Wang, Affiliated Hospital of North Sichuan Medical College, Nanchong, Sichuan, China, and colleagues analysed the interference factors causing false-positive reactivity using gold immunochromatography assay (GICA) and enzyme-linked immunosorbent assay (ELISA).
A total of 86 serum samples were used to detect SARS-CoV-2 IgM using both methods. Of the 86 samples, only 14 were from patients with coronavirus disease 2019 (COVID-19). The other samples were from patients with influenza, HIV, and diabetes, among others.
Both GICA and ELISA detected positive SARS-CoV-2 IgM in 22 middle-high level RF-IgM-positive sera and in all the 14 sera from the patients with COVID-19 patients. The other 50 sera were negative.
The researchers then used the urea dissociation test to dissociate the SARS-CoV-2 IgM-positive serum using the best dissociation concentration.
When urea dissociation concentration was 6 mol/L, SARS-CoV-2 IgM was positive in 1 middle-high level RF-IgM-positive sera and in the 14 COVID-19 samples detected using GICA. When urea dissociation concentration was 4 mol/L and the avidity index (AI) lower than 0.371 was set to negative, test results were positive for SARS-CoV-2 in 3 middle-high level RF-IgM-positive sera, as well as the 14 COVID-19 sera detected using ELISA.
“Middle-high level of RF-IGM could lead to false-positive reactivity of SARS-CoV-2 IgM detected using GICA and ELISA,” the authors concluded. “Urea dissociation tests would be helpful in reducing false-positive results of SARS-CoV-2 IgM.”
