Nucleocapsid antigen found to be a sensitive biomarker for acute COVID-19 infection

A new preprint study published in the medRxiv* preprint server found that nucleocapsid antigen in the blood (antigenemia) is a sensitive and specific biomarker for acute coronavirus disease 2019 (COVID-19) infection.

The study, led by Sean R. Stowell of Brigham and Women’s Hospital in Massachusetts, also found elevated levels of nucleocapsid antigenemia in samples without evidence of IgG and anti-spike seroconversation. Along with an association for infection, antigenemia is correlated with serostatus and disease severity.

“We conclude that nucleocapsid antigenemia is a promising candidate biomarker for 291 active viral replication – the definition of which is the presence of replication-competent virus in a host – 292 recognizing that the available evidence points to this being an individualized process that cannot be 293 broadly defined based on a timeline,” explained the researchers.

Identifying nucleocapsid antigenemia may help to identify people whose infections have ended or if there is continued persistence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). It also supports future work looking into additional biomarkers that may be used to identify signs of SARS-CoV-2 viral replications.

Study details

Between January 11, 2021, and March 12, 2021, the team studied blood, serum, and plasma samples collected from routine clinical testing from inpatient and outpatient medical settings. Duplicate blood samples from the same patient were included as long as there was a minimum of five days between specimen collection.

Researchers used serological screen assays detecting the SARS-CoV-2 receptor-binding domain (RBD) and nucleocapsid antibodies to determine a patient’s serological status at the time of antigenemia testing. Nucleocapsid antibody testing was performed with an ELISA.

A total of 2,367 serum and plasma samples from 2,101 different patients were tested for antigenemia. About 10.2% showed detectable nucleocapsid levels.

In the primary analysis, there were 2.1% of blood samples with detectable antigen levels even though these patients were never confirmed to have SARS-CoV-2. The researchers explain these patients likely were infectious and had a missed or false-negative diagnosis, further supporting a potential role for antigenemia screening.

Medical records were also obtained to record patient information on the use of mechanical ventilation, severity of symptoms, and date of death. Blood samples were labeled according to patients’ COVID-19 status level and category (convalescent, late-presenting, acute, preCOVID, and same-day negative).

Antigenemia is associated with COVID-19 infection

Plasma and serum samples from acute COVID-19 illness had higher levels of nucleocapsid antigenemia than samples labeled as late-presenting, convalescent, preCOVID, or same-day negative.

Having nucleocapid antigenemia in samples was 85.2% sensitive and 89.9% specific for acute infection. When excluding samples from people without COVID infection, sensitivity levels rose to 93.9%.

Ct values from positive COVID-19 tests were available for 49 specimens. Only 6 out of 17 with values greater than 33 had antigenemia. Every sample, except for two with Ct values less than 30, showed signs of antigenemia.

Antigenemia correlates with antibody serostatus and COVID-19 severity

Nucleocapsid levels differed from seronegative samples compared to seropositive samples for For nucleocapsid IgG, RBD IgG, RBD IgA, and RBD IgM.

Specifically, seropositive samples were more likely to have undetectable antigenemia.

Nucleocapsid antigen samples were higher in people with COVID-19 illness who died or needed intubation within 30 days of sampling compared to those who survived or did not need intubation.

Nucleocapsid antigenemia levels were not significantly associated with elevated D-dimer. However, they did correlate with elevated CRP levels.

Study limitations

The researchers note that recording of symptoms may have been subjective and prone to recall bias. Because of limitations in community-based testing, some patients may have had SARS-CoV-2 before getting an official test.

Nucleocapsid-specific immunoglobulin may obstruct measurements of antigenemia in individuals who have seroconverted. However, it remains unknown whether Ig-bound and unbound antigen or unbound antigens are a more meaningful clinical indicator.

The study also makes the assumption that each sample came from a patient who is immunocompetent and that each patient had a similar course of acute COVID-19. Additionally, there was an assumption that no other external factors could be contributing to antigenemia.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Jocelyn Solis-Moreira

Jocelyn Solis-Moreira graduated with a Bachelor's in Integrative Neuroscience, where she then pursued graduate research looking at the long-term effects of adolescent binge drinking on the brain's neurochemistry in adulthood.