Antibody Tests

There is no test for COVID-19.  Instead, testing focuses on detection of SARS-CoV-2, the virus known to cause COVID-19 or the body’s response to infection.  Today we will discuss antibody tests, the blood tests used to detect past infections.

Part of a healthy immune response to infection by any virus is the formation of antibodies.  Antibodies are made in plasma cells, a special type of lymphocyte, in response to an assault by a foreign pathogen such as a virus.  The immune response is amazing.  Once plasma cells “learn” how to identify the virus, they crank out millions of these little molecules which coat the surface of the virus, flagging them as “enemy”, and directing other cells of the immune system to isolate and eliminate them.  Once the infection is cleared, the plasma cells “remember” the code for that virus so they can be quickly recruited to make more antibody if the same infection occurs in the future.  This memory eliminates the time-consuming “learning” step.  This simple process explains why most people only get chickenpox once and explains how vaccines can protect children from mumps and measles. 

The first antibodies produced after infection are known as IgM Antibodies.  These are large pentamers, basically 5 antibody units brought together like a snowflake.  IgM antibody production is replaced with IgG antibody production generally after about a week, but the time varies by individual and virus.  IgG antibodies are produced for weeks to years after infection, again depending on the individual and the virus.  Other antibodies are produced by the immune system, but IgM and IgG are most useful in viral serology.  These antibodies are found in the blood stream, so a simple blood test can detect a variety of antibodies, including antibodies to SARS-CoV-2.  It is thought that antibodies to SARS-CoV-2 is what makes convalescent plasma an effective treatment for severe COVID-19.

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There are several facts to keep in mind when interpreting antibody test results.  First, antibody tests tell us about past infections, not active infections.  Second, the antibody form tells us whether the infection was recent or distant.  IgM antibodies are made first but go away, so detection of IgM antibodies means the infection was recent.  IgG antibodies are made last but stay around for a long time.  Detection of IgG without IgM means that the infection occurred in the more distant past, at least three weeks ago, and detection of both IgG and IgM means the infection occurred in the transition period.

Antibody tests use an immunoassay methodology, which basically uses antibodies to detect antibodies.  Many of these tests have good specificity and sensitivity, but they are not perfect.  Cross reactivity and interfering substances can cause false positive and false negative results in the best of circumstances.  But we are not in the best of circumstances.  All SARS-CoV-2 antibody tests are available in the U.S. by an emergency use agreement (EUA) with the FDA.  Rigorous studies demanded by the FDA approval process have not yet occurred.

So, if you are positive for SARS-CoV-2 antibody, will you always be?  Don’t know.  If you are positive for SARS-CoV-2 antibody are you immune to COVID?  Don’t know.  It is simply too soon to define the body’s usual response to infection and its implication for future infection.  

What can we learn from antibody tests?  If your antibody test is positive, you probably had a SARS-CoV-2 infection in the past, even if you do not remember being sick.  If you do not have symptoms, and you have not been around someone with COVID-19, then you are not likely to have a current infection.  Continue normal activities, but with vigilant precautions.  Wear a mask and social distance.  These precautions reduce the spread of the disease.