In my last post, I discussed the value of serologic antibody tests for COVID-19. Briefly, these tests detect the presence of IgM and IgG antibodies that a person builds to fight an infection. These tests can be used at a global scale to find out how many people have been infected with COVID-19, who potentially can return to work, and perhaps who we can retrieve convalescent plasma to treat active infection.
Today, the FDA provided its first emergency use authorization for serologic antibody testing to Cellex, Inc. Cellex, Inc is a biotechnology company that specializes in point-of-care diagnostics technology. They have previously developed high sensitive and specific immunochromatographic assays for various infectious, tropical, gastrointestinal, and respiratory diseases. For their COVID-19 antibody test, they use a lateral flow immunochromatographic assay to detect the presence of IgM and IgG antibodies.
Lateral Flow Test
Here is a great video demonstration of how a lateral flow immunoassay test works.
A drop of analyte (i.e. blood, plasma, serum) is applied to the sample pad. This drop will flow down the cellulose membrane through capillary mass action. The substances in the analyte then bind to their matching antibodies tagged with a marker that can be visualized or with control line antibodies. Matched antibodies are then captured and displayed at the test line of the immunoassay.
Cellex Test for COVID-19 Antibodies
Here is the Cellex brochure for their antibody test. The instructions are simple: add 10 microLiters of sample (blood, plasma, or serum), add two drops of sample diluent, and then wait 15 minutes for the result.
A negative result shows the presence of the C band without any color in the G or M band. There are several possibilities of positive results, shown below. If only G band is present, this suggests a late stage primary, early secondary, or past infection with COVID-19. If only the M band is active, this suggests a fresh primary coronavirus infection. If there are both M and G bands, this means there is a current primary or early secondary infection.
Providers are recommended that a negative test does not rule out COVID-19 infection and should not be used as a sole basis for treatment decisions.
Efficacy of Test
Cellex, Inc. tested the efficacy of their serological antibody test against 128 samples with SARS-CoV-2 infection confirmed with RT-PCR and 250 negative control samples. The sensitivity was 93.75% and specificity was 96.4%.
There was no cross reactivity with other viruses like HIV, influenza, or Rhinovirus. There was also no false positive findings with elevations in hemoglobin, bilirubin, cholesterol, rheumatoid factor, human IgG/IgM, or common antibiotics.
The FDA provided emergency use authorization stating that it is reasonable to believe that the Cellex, Inc antibody test may be effective in diagnosing COVID-19.
There are a ton of products now in the market that provide serologic antibody tests. However, all those products are required to state that they are not reviewed by the FDA, negative results do not rule out infection, and positive results can be contaminated from a past non COVID-19 related infection.
This makes it difficult for any healthcare provider to determine which test to purchase. This is especially important because Spain had to recall 58,000 ordered tests after learning they had poor accuracy.
However, FDA EUA authorization gives more credence and confidence to purchase Cellex products..