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Fast, Inexpensive Covid-19 Immunity Test Devised

Drop of blood

(Alden Chadwick, Wikimedia Commons)

22 Mar. 2021. Researchers in Canada developed a simple and inexpensive test for Covid-19 antibodies requiring a pinprick of blood, and returning results in under an hour. A team from the lab of biochemistry professor Igor Stagljar at University of Toronto describes the test and results with human blood samples in today’s issue of the journal Nature Communications.

A continuing need for public health authorities during the Covid-19 pandemic is tracking the extent of immunity against infection in the population. Because the severity of disease in survivors ranges from severe symptoms to none at all, and as the extent of vaccination grows, health authorities need a quick and easy way for residents to gauge their immunity to infection. Likewise, many individuals may want to test their immunity status before travel or family gatherings, and a quick and inexpensive lab test would help make that possible.

The so-called gold standard today to check for antibodies in blood is tests using an enzyme-linked immunosorbent assay or Elisa technology. That assay, a system found in most licensed diagnostic labs, identifies and measures the number of antigens in blood samples. But an Elisa-based system usually requires multiple steps in a full-scale lab, trained staff, and usually several hours to complete and report. Other alternatives are paper strip tests like home pregnanacy tests that provide results quickly, but according to the researchers are not considered as reliable as Elisa assays, and give only a yes/no response, not a measurable quantity.

Stagljar’s lab studies properties of proteins found on cell membranes, which comprise about one-third of all proteins in cells, including receptor proteins that interact with other proteins from outside the cell. Up to last year, much of the lab’s work was devoted to cancer, but with the pandemic the lab turned to the SARS-CoV-2 virus responsible for Covid-19 infections. The result of their work is a test called SATiN, short for Serological Assay based on split Tripart Nanoluciferase. SATiN uses illuminating properties of luciferase enzymes, the same biochemicals that cause fireflies to illuminate.

Cost less than two Canadian dollars per test

A SATiN test uses a brightly illuminating luciferase enzyme called NanoLuc, divided chemically in three parts. One part is bound to the S protein found on the surface of SARS-CoV-2 spikes, with another part fixed to protein G, found on bacterial cell walls that binds to immunoglobulin G or IgG antibodies found in blood serum. The third chemical segment remains unattached, which without this segment, prevents the luciferase enzyme from lighting up.

When IgG antibodies appear, however, the binding of an antibody with the spike protein and protein G links the missing luciferase segment with the other two parts, enabling the enzyme to illuminate, indicating the presence of IgG antibodies in the sample. The illumination can be read on a standard microplate reader for luminescent tests found in many labs, with results returned in under an hour, according to the researchers. And the team says the test costs less than two Canadian dollars, about $US 1.60.

A team led by test developer and first author Zhong Yao, senior research associate in the lab, tested SATiN on human blood samples, and compared those results to an Elisa-based test evaluating the same samples. The researchers assessed their test with drops of seven blood serum samples stored from before the pandemic and 82 Covid-19 samples from patients across Canada taken up to 80 days after symptom onset, to get variable concentrations of antibodies.

The results show the SATiN detects antibodies in blood serum samples, with the highest concentrations in samples taken within 20 days of symptom onset. And SATiN results correlated highly with Elisa-based lab tests, including concentration levels, offering a potential tool for measuring antibody levels and verifying vaccination status. “This will be of crucial importance for the next stage of the pandemic,” says Stagljar in a university statement, “especially now when governments of all countries started with mass vaccinations with recently approved anti-COVID-19 vaccines.”

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