A preliminary study on serological assay for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in 238 admitted hospital patients

Abstract

In this study, we aimed to evaluate the diagnostic value of serological assay for SARS-CoV-2. A newly-developed ELISA assay for IgM and IgG antibodies against N protein of SARS-CoV-2 was used to screen the serums of 238 admitted hospital patients between February 6 and February 14, 2020 with confirmed or suspected SARS-CoV-2. SARS-CoV-2 RNA was detected on pharyngeal swab specimens using real time RT-PCR. 194 (81.5%) of the serums were detected to be antibody (IgM and/or IgG) positive, significantly higher than the positive rate of viral RNA (64.3%). There was no difference in the positive rate of antibodies between the confirmed patients (83.0%, 127/153) and the suspected patients (78.8%, 67/85), whose nucleic acid tests were negative. The antibody positive rates were very low in the first five days after initial onset of symptoms, and then rapidly increased as the disease progressed. After 10 days, the antibody positive rates jumped from below 50% to over 80%. However, the positive rates of viral RNA maintained above 60% in the first 11 days after initial onset of symptoms, and then rapidly decreased. Overall, the suspected patients were most likely infected by SARS-CoV-2. Before the 11th day after initial onset of symptoms, nucleic acid test is key for confirmation of viral infection. The combination of serological assay can greatly improve the diagnostic efficacy. After the 11th day post-disease onset, the diagnosis for viral infection should be majorly dependent on serological assay.

Keywords: Diagnosis; Nucleic acid test; SARS-CoV-2; Serological assay.

Fig. 1

Positive rate of viral RNA and antibody in different samples. A) The positive rate of viral RNA (black column) and antibody (white column) in 238 enrolled patients (two columns on the left), as well as the positive rate of antibody in ordinary patients and healthy donors (two columns on the right). B) Comparison of positive rate of antibody between the laboratory-confirmed (left) and highly-suspected patients (right). Results were compared by chi-square tests.

Fig. 2

Dynamics of the positive rate of viral RNA and antibody of the patients at the different stages of disease. The disease courses were divided into five phases of 0–5, 6–10,11-12, 13–15 days and more than 16 days after initial onset of symptoms. The positive rate of viral RNA (solid circle) and antibody (hollow circle) of the patients at the different phase of disease was shown.