Time course of the sensitivity and specificity of anti-SARS-CoV-2 IgM and IgG antibodies for symptomatic COVID-19 in Japan

Abstract

The accurate and prompt diagnosis of SARS-CoV-2 infection is required for the control and treatment of the coronavirus infection disease 2019 (COVID-19). In this study, we aimed to investigate the time courses of the anti-severe acute corona respiratory syndrome coronavirus 2 (SARS-CoV-2) IgM and IgG titers and to evaluate the sensitivity and specificity of such tests according to the specific day after the onset of COVID-19 among a patient population in Japan. We measured the titers of SARS-CoV-2 IgM and IgG in sera from 105 subjects, including 26 symptomatic COVID-19 patients, using chemiluminescent immunoassay (CLIA) methods utilizing magnetic beads coated with SARS-CoV-2 nucleocapsid protein and spike protein. The results of a ROC analysis suggested the possibility that the cutoff values in Japan might be lower than the manufacturer's reported cutoff (10 AU/mL): 1 AU/mL for IgM and 5 AU/mL for IgG. The sensitivity of the test before Day 8 after symptom onset was less than 50%; at Days 9-10, however, we obtained a much higher sensitivity of 81.8% for both IgM and IgG. At 15 days or later after symptom onset, the SARS-CoV-2 IgG test had a sensitivity of 100%. These results suggest that if the number of days since disease onset is taken into consideration, these antibody tests could be very useful for the diagnosis of COVID-19 and similar diseases.

Figure 1

The time courses of serum titers of SARS-CoV-2 antibodies in COVID-19 patients after symptom onset. We measured the titers of SARS-CoV-2 IgM and SARS-CoV-2 IgG in 125 sera collected from RT-PCR-positive patients (n = 26) and 68 sera collected from RT-PCR-negative patients (n = 35). The data are plotted for days 1–2, 3–4, 5–6, 7–8, 9–10, 11–12, 13–14, 15–16, 17–18, 19–20, and ≥ 21 since symptom onset. The bars show the median titers at each timepoint. The broken line shows the manufacturer’s cutoff value (10 AU/mL). (A) RT-PCR-positive patients, IgM. (B) RT-PCR-positive patients, IgG. (C) RT-PCR-negative patients, IgM. (D) RT-PCR-negative patients, IgG.

Figure 2

ROC curves for SARS-CoV-2 antibody tests. We compared the discriminating abilities of the SARS-CoV-2 IgM and SARS-CoV-2 IgG titers at different time points after symptom onset. The cutoff value for both tests was set at 10 AU/mL. (A) SARS-CoV-2 IgM, (B) SARS-CoV-2 IgG.

Figure 3

Diagnostic abilities of SARS-CoV-2 antibody tests according to cutoff values and time courses. We compared the sensitivities and specificities for the serum diagnosis of COVID-19 each cut off value (1–10 AU/mL) at 9–10 days after symptom onset (A: SARS-CoV-2 IgM, B: SARS-CoV-2 IgG) and investigated the time course of the diagnostic ability when the cut off values were set as 1, 2, 5 and 10 AU/mL (C: 10 AU/mL, D: 5 AU/mL, E: 2 AU/mL, F: 1 AU/mL).

Figure 4

Comparison of the specificity against “RT-PCR-negative group” with that against “Before COVID-19 group”. We compared the specificity against “RT-PCR-negative group” (n = 79) with that against “Before COVID-19 group” (n = 100), which contains healthy subjects in 2017. (A) SARS-CoV-2 IgM, (B) SARS-CoV-2 IgG.