False Positivity of Anti-SARS-CoV-2 Antibodies in Patients with Acute Tropical Diseases in Thailand

Abstract

Serology remains a useful indirect method of diagnosing tropical diseases, especially in dengue infection. However, the current literature regarding cross-reactivity between SARS-CoV-2 and dengue serology is limited and revealed conflicting results. As a means to uncover relevant serological insight involving antibody classes against SARS-CoV-2 and cross-reactivity, anti-SARS-CoV-2 IgA, IgM, and IgG ELISA, based on spike and nucleocapsid proteins, were selected for a fever-presenting tropical disease patient investigation. The study was conducted at the Faculty of Tropical Medicine during March to December 2021. The study data source comprised (i) 170 non-COVID-19 sera from 140 adults and children presenting with acute undifferentiated febrile illness and 30 healthy volunteers, and (ii) 31 COVID-19 sera from 17 RT-PCR-confirmed COVID-19 patients. Among 170 non-COVID-19 samples, 27 were false positives (15.9%), of which IgA, IgM, and IgG cross-reactive antibody classes were detected in 18 (10.6%), 9 (5.3%), and 3 (1.8%) cases, respectively. Interestingly, one case exhibited both IgA and IgM false positivity, while two cases exhibited both IgA and IgG false positivity. The false positivity rate in anti-SARS-CoV-2 IgA and IgM was reported in adults with dengue infection (11.3% and 5%) and adults with other tropical diseases (16.7% and 13.3%). The urea dissociation method applied to mitigate false positivity resulted in significantly decreased ELISA-based false and true positives. In conclusion, the analysis of antibody against SARS-CoV-2 in sera of patients with different tropical diseases showed that high IgA and IgM false positivity thus potentially limits serological assay utility in fever-presenting patients in tropical areas.

Keywords: COVID-19; ELISA; SARS-CoV-2; Thailand; acute febrile illness; antibodies; cross reaction; dengue; false positive reaction; tropical diseases.

Figure 1

Serum samples used in the study. A-AUFI, adults with undifferentiated febrile illness; C-AUFI, children with acute undifferentiated febrile illness.

Figure 2

Anti-SARS-CoV-2 ELISA result distribution in each group. Analysis of anti-SARS-CoV-2 ELISA of IgA (A), IgM (B), and IgG (C) in serum samples from COVID-19, dengue, adults, and children patients with acute undifferentiated febrile illness (AUFI) and healthy persons. ns, non-significant; *, p < 0.05; ****, p < 0.0001.

Figure 3

The anti-SARS-CoV-2 antibodies in correspondence with urea dissociation. Urea dissociation was performed with all borderline and positive results for anti-SARS-CoV-2 ELISA, both false positive (A–C) and true positive (D–F). The results of anti-SARS-CoV-2 IgA (A,D), IgM (B,E), and IgG (C,F) prior and subsequent to urea dissociation were plotted. An OD ratio <0.8 was considered as negative, 0.8–1.1 was considered as borderline (red horizontal area), and ≥1.1 was considered positive.