Coronavirus antibody tests could be wrong and can't be used to allow people to return to work, warns CDC
Stating that antibody tests could be inaccurate many times, the US Centers for Disease Control and Prevention (CDC) has warned that such tests should not be used to make important policy decisions such as returning to workplaces or discontinuing physical distancing during the Covid-19 pandemic. Antibody tests, also referred to as serologic tests, use blood samples to see if a person’s immune system has created antibodies to fight Covid-19. They allow doctors to understand if a person has been exposed to the virus in the past.
“Serologic test results should not be used to make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities. Serologic test results should not be used to make decisions about returning persons to the workplace,” says the CDC in the new guidance.
The advisory comes even as the CDC recently admitted that it has been combining Covid-19 results from two different kinds of tests - diagnostic tests and antibody tests - when reporting the country's testing totals. Diagnostic tests take nose swabs or saliva samples and they look for direct evidence that someone currently has Covid-19. Combining their results with those of antibody tests may give an incorrect picture of when and where coronavirus spread as the US eases lockdown.
In its latest guidelines, the agency says that health officials or health care providers who are using antibody tests need to use the most accurate test they can find and they might need to test people twice. In most of the country, including areas that have been heavily impacted, the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from less than 5% to 25%, so that testing at this point “might result in relatively more false-positive results and fewer false-negative results,” suggests data. Therefore, says the CDC, it is important to minimize false-positive results by choosing tests with high specificity and by testing populations and individuals with a higher likelihood of previous exposure to SARS-CoV-2.
The CDC explains why testing can be wrong so often and a lot has to do with how common the Covid-19 virus is in the population being tested. “In the current pandemic, maximizing specificity and thus positive predictive value in a serologic algorithm is preferred in most instances since the overall prevalence of antibodies in most populations is likely low. For example, in a population where the prevalence is 5%, a test with 90% sensitivity and 95% specificity will yield a positive predictive value of 49%. In other words, less than half of those testing positive will truly have antibodies,” explains the agency. It adds, “Alternatively, the same test in a population with an antibody prevalence exceeding 52% will yield a positive predictive greater than 95%, meaning that less than one in 20 people testing positive will have a false-positive test result.”
Experts have cautioned that people who assume that they are immune to a second infection because they have received a positive test result may ignore public health advice. The World Health Organization (WHO) had earlier warned that there is currently “no evidence” that people who have recovered from Covid-19 and have antibodies are protected from a second coronavirus infection. Accordingly, the WHO warned countries against issuing “risk-free certificate” or “immunity passport” to infected people on the assumption they would be immune to reinfection as their accuracy cannot be guaranteed.
The new CDC advisory is on similar lines as the WHO. The experts say antibody tests should not be used at this time to determine if an individual is immune. The document explains that while the presence of anti-SARS-CoV-2 antibodies “when detected using a testing algorithm with high positive predictive value for the context of use likely indicates at least some degree of immunity,” until the durability and duration of immunity are established, it cannot be assumed that individuals with truly positive antibody test results are protected from future infection.
According to the agency, antibodies most commonly become detectable 1 to 3 weeks after symptom onset, at which time evidence suggests that infectiousness likely is greatly decreased and that some degree of immunity from future infection has developed. However, definitive data are lacking, and it remains uncertain whether individuals with antibodies are protected against reinfection with SARS-CoV-2, and if so, what concentration of antibodies is needed to confer protection. “Additional data are needed before modifying public health recommendations based on serologic test results, including decisions on discontinuing physical distancing and using personal protective equipment,” says the CDC.
Accordingly, the CDC says that persons who have had a “Covid-19-compatible or confirmed illness” should follow previous guidance regarding the resumption of normal activities, including work. There should be no change in clinical practice or use of personal protective equipment (PPE) by health care workers and first responders who test positive for SARS-CoV-2 antibody, recommends the CDC. “Asymptomatic persons who test positive by serologic testing and who are without recent history of a Covid-19 compatible illness have a low likelihood of active infection and should follow general recommendations to prevent infection with SARS-CoV-2 and otherwise continue with normal activities, including work,” say guidelines.