Study demonstrates utility of serum antibody testing for COVID-19
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Researchers found that serum testing for SARS-CoV-2 antibodies performs well when used 2 weeks or more after symptom onset.
The antibodies, which were associated with clinical severity in patients with COVID-19, demonstrated infection up to 2 months after symptom onset, they said. Previously, experts questioned the utility of antibody tests, but the results show that they can be used to complement the gold standard of COVID-19 diagnosis with nucleic acid amplification tests (NAATs) on nasopharyngeal swabs.
![Title: Performance of serum antibody testing for SARS-CoV-2 when used 14 days after symptom onset: [Treatment Group A] Sensitivity: 97.6% [Treatment Group B] Specificity 98.8%](/~/media/slack-news/fm_im/misc/infographics/2020/august/pc0820caturegli_graphic_01.jpg?w=800)
The findings “provide valuable diagnostic support in patients who test negative by NAAT but remain clinically suspicious for COVID-19,” Patrizio Caturegli, MD, MPH, an associate professor of pathology at the Johns Hopkins University School of Medicine, and colleagues wrote in Annals of Internal Medicine.
In a case-control study, Caturegli and colleagues analyzed results from 115 hospitalized adults who were tested for COVID-19 using serum and NAAT. Sixty of these patients tested positive (case patients) and 55 did not (controls). An additional 513 individuals who were not tested by NAAT but provided serum samples to the lab were also included in the analysis.
All serum samples were tested for the presence of immunoglobulin G and immunoglobulin A antibodies against SARS-CoV-2 using an enzyme-linked immunosorbent assay.
According to the researchers, antibodies to SARS-CoV-2 begin to appear about 10 days after symptom onset and “rapidly increase to a high and sustained plateau” — demonstrating infection for up to 58 days. The sensitivity of the serum assay was 0.976 (95% CI, 0.928–0.995) and its specificity was 0.988 (95% CI, 0.974–0.995) when measured at least 14 days after symptom onset. However, its sensitivity decreased before the 2-week mark, they said. Antibodies to SARS-CoV-2 predicted the odds of developing acute respiratory distress syndrome (ARDS), which increased by 62% (95% CI, 48–81) for every twofold increase in IgG.
According to the researchers, NAAT and antibody results showed agreement in 102 of the 115 study patients (both tests were positive in 46 patients) for an overall concordance of 89%.
In a larger cohort of 11,66 tested patients with suspected COVID-19, 457 were repeatedly negative using NAAT. Among 18 of these patients who also received serum testing, antibodies were present in five of six case patients and in none of the 12 control patients (P = .001).
“Cases where clinical suspicion remains high despite repeated negative NAAT results could especially benefit from serologic testing,” Caturegli and colleagues wrote. “Our study revealed a strong relationship between antibodies and development of ARDS in COVID-19. This novel finding provides further insights on the contribution of the immune system to COVID-19 pathogenesis, as demonstrated in cytokine storm.”
Perspective
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Maria Luisa Alcaide, MD
The timing of this study is critical. Caturegli and colleagues were able to show that immunoglobulin G was detected in most participants up to 58 days, indicating how long antibodies will persist after infection with SARS-CoV-2. Researchers also found that the level of the antibody response correlated with the severity of the disease. Though it is still too early to ascertain the long-term significance of these findings, they are still very timely and important.
Antibody testing determines whether somebody has been infected with SARS-CoV-2, and may predict the risk for others to be infected may protect against a new infection. Though our knowledge in these areas is limited for COVID-19, with other viruses, we know that antibody responses last for a long time, protecting those with the antibodies from future such infections.
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