June 25 (UPI) -- Antibody testing might help determine whether people have had COVID-19, but its effectiveness depends on when the test occurs, according to an analysis published Thursday by the Cochrane Database of Systematic Reviews.
In general, the tests were better at detecting COVID-19, the disease caused by the new coronavirus, SARS-CoV-2, two or more weeks after the onset of symptoms, the reviewers found.
It's unclear, however, how effective tests are in those checked more than five weeks after symptoms first appeared, researchers said.
"We do not know if this is true for people who have milder disease or no symptoms, because the studies in the review were mainly done in people who were in hospital," the researchers said in a press release.
"In time, we will learn whether having previously had COVID-19 provides individuals with immunity to future infection," they said.
Antibody tests have emerged as a potentially useful public health tool to identify individuals with previous COVID-19, according to the Cochrane database.
Findings can be used to gauge disease spread, and might serve as the backbone of studies designed to assess the use of blood plasma with antibodies against the virus as a treatment for those currently infected, researchers wrote in the analysis.
Once peeople are infected with the new coronavirus, their immune system responds by developing proteins in the blood called antibodies that attack the virus. Detecting antibodies in people's blood may indicate whether they have COVID-19 or have had it previously, according to Cochrane.
Cochrane researchers from universities across the world, led by those from the University of Birmingham in England, analyzed more than 11,000 peer-reviewed studies on COVID-19 published before April 30.
The researchers looked for reported results of antibody tests in groups of people known to have had COVID-19, as well as others known not to have had COVID-19.
They found a total of 54 relevant studies reporting test results for about 16,000 samples. The majority of studies were from China, and were carried out in hospitalized patients mostly likely with severe disease.
Data were only available for 27 of the more than 200 antibody tests on the market -- including lab-based tests, in which blood samples are taken from veins, and point-of-care tests, which use finger-prick blood samples -- researchers said. There was insufficient data to compare the accuracy of various tests, they said.
Overall, tests for two specific types of antibodies performed eight to 14 days after symptom onset correctly identified only 70 percent of people who had COVID-19, they found. However, at 15 to 35 days after symptom onset, the tests accurately detected more than 90 percent of people with COVID-19, they said.
In addition, the tests wrongly diagnosed COVID-19 in 1 percent to 2 percent of people without the disease, they added.
"We've analyzed all available data from around the globe -- discovering clear patterns telling us that timing is vital in using these tests," Jon Deeks, one of the researchers who worked on the review, said in a statement.
"Use them at the wrong time and they don't work," said Deeks, a professor of Biostatistics and head of the Test Evaluation Research Group at the University of Birmingham.
Deeks and colleagues also had several concerns about the quality of the studies they reviewed because most were small and did not report results fully. Many papers also included multiple samples from the same patients and more than half were made available before they had been through peer review, they said.
The researchers plan to update their review over the next few months to provide a more complete summary of the research evidence as it accumulates.
"This is a fast-moving field and we plan to update this review regularly as more studies are published," said Jac Dinnes, who is part of the University of Birmingham team.
Cochrane Reviews is a database of systematic reviews and meta-analyses which summarize and interpret the results of medical research.