A new type of SARS-CoV-2 antigen test that relies on single molecule array technology may be able to help clinicians identify which patients are most likely to experience severe disease.

A recent study published in Clinical Chemistry found a high correlation between blood levels of S1 – a protein that is part of the SARS-CoV-2 viral structure – and admissions to the intensive care unit (ICU). The study tested the use of an ultra-sensitive antigen test that assesses antigen levels in the plasma of patients at the single molecule level, making its sensitivity comparable to molecular tests.

“This is the first paper where we are measuring antigens in the blood. Unlike everything you’re reading about antigen tests, which are aimed at saliva or nasopharyngeal swabs, we’re actually measuring antigens in the blood and that’s enabled by the ultra-sensitivity of these tests,” David Walt, a pathology professor at Brigham and Women’s Hospital and a professor at Harvard Medical School, both in Boston, told BioWorld.

The Single Molecule Array (Simoa) SARS-CoV-2 antigen test would likely be conducted under the US FDA’s laboratory-developed test guidelines, according to Walt.

The test is not currently designed to be a point-of-care diagnostic like most of the antigen tests that have emergency use authorization from the FDA. But it could be used in the future in conjunction with molecular RT-PCR testing to alert clinicians when patients need closer monitoring in the hospital or are likely to require greater clinical intervention, such as a ventilator.

Antigens in plasma

David Walt, lead author on the Clinical Chemistry paper

In the study, Walt and his colleagues used single molecule array (Simoa) assays to quantitatively detect SARS-CoV-2 spike, S1 subunit, and nucleocapsid antigens in the plasma of patients who had tested positive for COVID-19 in RT-PCR tests after presenting to the emergency department. Patients who were COVID-19 negative and pre-pandemic patient samples were also analyzed for comparison.

Among 64 patients who were positive for COVID-19, the researchers detected S1 and nucleocapsid in 41 patients. When evaluating how the antigens correlated with clinical outcomes, such as ICU admission and the need for intubation, the researchers found that S1 had a higher correlation with clinical severity than nucleocapsid in plasma.

Looking deeper at the link with S1, they found that 23 patients had undetectable S1 concentrations, 23 patients had low concentrations, and 18 patients had high concentrations. There was a statistically significant difference in rates of ICU admission for those three groups based on the level of concentration (P = 0.0107).

Patients with undetectable levels of S1 concentration were admitted to the ICU upon presentation to the hospital at a rate of 30%. That rate climbed to 52% for patients with a low concentration of S1, and 77% for patients with a high concentration.

The researchers also reported a statistically significant difference in the time to intubation between patients with high S1 concentration and those with no detectable S1 concentrations (P = 0.0050). “These results suggest that high S1 concentrations in plasma upon presentation to the hospital correlate with severe cases of COVID-19 that can result in respiratory failure and require immediate intubation,” the researchers wrote in Clinical Chemistry.

“Our hypothesis is that the mechanism by which the antigen is getting into the blood is through damage to endothelial cells – that is the lining of the lungs, which is where the infection initially takes place -- and that those individuals who have more severe damage to their lungs and respiratory systems pass more of the viral proteins through those damaged tissues into the bloodstream,” Walt said. “That’s what’s causing the need for them to be intubated and ventilated.”

Along with the antigen findings in plasma, the researchers also used the Simoa assays in saliva samples. They tested saliva samples from patients who presented to the emergency department at Brigham and Women’s Hospital who tested positive by RT-PCR for COVID-19. They were able to detect S1 and nucleocapsid in 7 of 11 samples. While that testing sample is too small to draw definitive conclusions, Walt said it indicates that the test has the potential to be adapted for use in saliva, positioning it as a diagnostic tool.

Research tool

While the Simoa assay doesn’t presently look like a potential point-of-care diagnostic tool due to its complexity, the study provides some important information about the behavior of the SARS-CoV-2 virus, according to Geoffrey Baird, professor and interim chair of the department of laboratory medicine and pathology at the University of Washington, Seattle.

“This is another tool in the research approach to understanding the biology and pathophysiology of the coronavirus,” Baird said.

For starters, this is the first study to detect viral antigens in the plasma of COVID-19 positive patients. Additionally, the technique could be used to help answer lingering questions about the course of the virus, specifically the amount of viral load among asymptomatic versus symptomatic patients, as well as how that viral load rises and falls over the course of illness.

“We don’t actually know what that curve looks like, so we could actually make that curve,” Baird said.