Skip to main content
Premium Trial:

Request an Annual Quote

SARS-CoV-2 Serology Assay Evaluations Show Seropositivity Increases With Time After Symptom Onset

NEW YORK – An evaluation of a dozen SARS-CoV-2 serology assays has highlighted a number of variables in test performance including increasing seropositivity with increasing time from symptom onset and a range of test specificities. 

Dozens of serology assays measuring antibodies against SARS-CoV-2 have received Emergency Use Authorization from the US Food and Drug Administration. Such assays are intended to gauge whether a particular patient has been exposed to SARS-CoV-2 and better enable estimates of viral prevalence and incidence.  

To assess the performance of these tests, researchers from the University of California, San Francisco compared 10 point-of-care lateral flow assays (LFAs) and two lab-based ELISA tests to detect anti-SARS-CoV-2 IgM and IgG antibodies in samples from patients with PCR-confirmed COVID-19 at different points in their disease course. They also evaluated the tests on samples from patients with other respiratory diseases, and in blood donation samples from before the start of the pandemic. 

The number of seropositive results typically increased with time from patients' symptom onset, with seropositivity peaking after more than 20 days. Four tests reached more than 80 percent positivity 16 days or more after symptom onset with more than 95 percent specificity, as the researchers reported in Nature Biotechnology on Thursday.

"We hope these data will inform the use of serology by the medical and public health communities and provide feedback to test developers about areas of success and necessary improvement," senior author Alexander Marson and his colleagues wrote in their paper.

The researchers compared the performance of the assays using 128 plasma or serum samples from 79 individuals who tested positive for SARS-CoV-2 by RT-PCR. Most patients presented with cough and fever.

A portion of the samples were obtained in the first five days after patients experienced symptoms, while others were collected further into their disease course, including samples from more than 20 days after symptom onset. The researchers also collected samples for analysis from 51 people with non-COVID-19 respiratory infections during the pandemic and from 108 pre-pandemic blood donor samples.

As there is no gold standard against which to benchmark the serological assays, the researchers assessed the assays' positive percent agreement with RT-PCR results. The number of samples that tested positive increased with time from first symptoms, with the highest percent positivity observed in samples from the 16-to-20-day time interval and in the more-than-20-days interval. Four assays — from Bioperfectus, Premier, Wondfo, and an in-house ELISA test — had more than 80 percent positivity at those time frames with more than 95 percent specificity. 

Based on the 108 pre-COVID-19 blood donor samples, the researchers reported that test specificity ranged from 84.3 percent to 100 percent. Most tests had a specificity of more than 95 percent, though two LFAs from Wondfo and Sure Biotech and the in-house ELISA had a specificity of more than 99 percent.

They further noted that, across all serology assays, IgM detection was less consistent than IgG detection.

A similar study conducted at Massachusetts General Hospital likewise uncovered increased detection of SARS-CoV-2 antibodies with increased time from symptom onset, the researchers noted.

Some samples that were from before the COVID-19 outbreak or that tested negative via PCR tested positive on serology assays, suggesting non-specific binding, cross-reactivity, or missed COVID-19 diagnoses.

Still, the findings suggested to the researchers that the assays they evaluated all have good to excellent sensitivity to detect anti-SARS-CoV-2 antibody among patients three or more weeks into their disease course. They added that further studies examining testing performance on ambulatory and asymptomatic individuals will aid in guiding the use of serology testing. 

The researchers cautioned, though, that it is still unknown where a positive serology result reflects a protective immune response and how long that protective response might last.