The U.S. Food and Drug Administration has granted Cellex an emergency use authorization to market a rapid antibody test for COVID-19, the first antibody test released amidst the pandemic.
“It is reasonable to believe that your product may be effective in diagnosing COVID-19,” and “there is no adequate, approved, and available alternative,” the agency said in a letter to Cellex.
A drop of serum, plasma, or whole blood is placed into a well on a small cartridge, and the results are read 15-20 minutes later; lines indicate the presence of IgM, IgG, or both antibodies against the SARS-CoV-2 virus.
Of 128 samples confirmed positive by reverse transcription polymerase chain reaction in premarket testing, 120 tested positive by IgG, IgM, or both. Of 250 confirmed negative, 239 were negative by the rapid test.
The numbers translated to a positive percent agreement with RT-PCR of 93.8% (95% CI: 88.06-97.26%) and a negative percent agreement of 96.4% (95% CI: 92.26-97.78%), according to labeling.
“Results from antibody testing should not be used as the sole basis to diagnose or exclude SARS-CoV-2 infection,” the labeling states.
Negative results do not rule out infection; antibodies might not have had enough time to form or the virus could have had a minor amino acid mutation in the epitope recognized by the antibodies screened for in the test. False positives can occur due to cross-reactivity with antibodies from previous infections, such as from other coronaviruses.
Labeling suggests that people who test negative should be checked again in a few days, and positive results should be confirmed by other methods. Also, the intensity of the test lines do not necessarily correlate with SARS-CoV-2 antibody titers.
As part of its authorization, the FDA waived good manufacturing practice requirements, but stipulated that advertising must state that the test has not been formally approved by the agency.
Testing is limited to Clinical Laboratory Improvement Amendments-certified labs. Positive results are required to be reported to public health authorities. The test can be ordered through Cellex distributors or directly from the company.
IgM antibodies are generally detectable several days after the initial infection, while IgG antibodies take longer. It’s not known how long COVID-19 antibodies persist after the infection has cleared, the agency said.