Why the focus on Covid19 serology testing and what does it mean?

In the midst of a truly overwhelming global pandemic, experts and novices alike are pushing for 'more testing, more testing.'  Since the middle of March, Maternova has been offering immunoassay rapid point of care tests.  

This blog attempts to greatly simplify the 'testing' debate and to simplify some of the scientific terms.  We don't intend the blog or the Maternova content to give epidemiological or medical advice, however.  We do wish that both the media and politicians would develop a more sophisticated language to describe what they mean by testing. 

PCR testing is for many scientists, a gold standard.  PCR can measure the actual presence of the virus in a swab (of mucous).  In the U.S. these are the only tests that were used or available for the first 6-8 weeks of Covid19.  The PCR test can be fairly rapid, now as little as 15 minutes' turnaround.  However, it does require an analyzer and several steps, as well as a skilled technician, to orchestrate the results of the test.  We won't discuss sensitivity and specificity here as it varies across tests.   The PCR test is ideal for identifying people who are in an early and active stage of the infection, because it is identifying the actual virus's genetic material rather than the body's immune response to the virus. 

By contrast, the immunoassay IgG/IgM test is a rapid point of care blood (or serology) test that is best suited for determining whether someone has, in the recent past, been exposed to, and mounted an immune response to the Covid19 infection. This test is made by immobilizing the antigen-- some of the actual protein of the virus (or a lab grown replica)-- on the rapid test (often using colloidal gold as part of the process). When the blood or serum of the patient is taken and directly put into the well at one end of the test, it travels along until it hits this immobilized protein, the test turns color (and the line appears at the IgG or IgM line or both).   The immunoassay tests are ideal for the following purposes:

1.  Determining who has actually had the Covid19 infection approximately 7 or more days previously (the exact day of infection at which the test works depends upon the test and the person)

2.  Determining infection rates and herd immunity-- if large numbers of people can be tested we may be able to determine that 50 or 60% of certain populations already have antibodies and decisions could be made about potential herd immunity. 

Drawbacks of the immunoassays include the following:

1.  For immunocompromised individuals whose bodies don't mount an antibody reaction, this kind of test will not accurately state whether they had Sars-CoV-19.

2.  Other kinds of false negatives-  If for example, someone is in the first days of the infection, the body may not have antibodies and the immunoassay would falsely describe he person as 'negative'

3.  False positives- the test might 'read' that the body had antibodies to other kinds of SARS viruses and thus read as a 'false positive' when in fact the person had not had Sars-CoV-19.  

Any testing decisions and protocols should be made in consultation with epidemiologists with explicit experience in infectious disease and with knowledge of the pros and cons of different types of tests.