By Alyssa Schnugg
The Mississippi Department of Health recently confirmed that daily updates on the number of positive COVID-19 cases can include positive antibody tests; however, not all positive results are counted.
As of Monday, 183,994 people have been tested for COVID-19. Of those, 7,740 have been tested for antibodies – about 4 percent of the total number of tests performed. Of the 15,752 positive cases reported Monday, about 80-90 people have tested positive for the presence of antibodies.
COVID-19 IgG antibody testing, also known as serology testing, checks for a type of antibody called immunoglobulin G (IgG).
According to Mayo Clinic, if test results show the presence of antibodies, it indicates the person was likely infected with COVID-19 at some time in the past.
It can take at least two weeks after exposure to develop antibodies.
State Epidemiologist Dr. Paul Byers said that some, but not all, positive antibody tests are counted as positive cases, based on the standard case definition from the CDC.
“Antibody tests or serology tests can determine prior infection but are less specific, and additional investigation is required to determine whether a positive serology is truly a case or not,” he told Hottytoddy.com recently. “Further investigation is initiated to identify symptoms consistent with COVID-19 or known close contact with a case.”
Byers said the standards for reporting cases of COVID-19 are the same that are in place for the reporting of any cases of any disease that is investigated.
“Positive tests require investigation to determine case status, based on published CDC/CSTE case classification guidelines, and these cases are then reported publicly if they meet the case definition,” he said. “Different test methodologies (serology, PCR) have different performance characteristics and not all positive results are equal.”
Urgent Care Clinic of Oxford’s Dr. David Coon said where the antibody testing is done is important when it comes to results. He said at his clinic, he has seen very few positive antibody tests.
“Our reference lab in Memphis has shown that their test has a very high percentage of positive test results in known COVID patients,” Coon said. “But keep in mind their test population have all had COVID proven by PCR testing. So, 100% of their test subjects had COVID. You’d expect a decent serology test to be close to 100% in that population.”
The problem, Coon said, arises when serology testing is done in a population of patients who are very unlikely to have had it, like in Oxford, where the incidence in the general population is very low.
Lafayette County has had 137 positive COVID-19 cases since March.
“In those cases, a positive test is much more likely to represent a false-positive result where the test is positive, but the patient never had COVID,” he said. “You would not want those included in daily counts. I presume that’s why they are counting positive serology patients only when there are other factors that corroborate a previous COVID infection.”