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Yesterday afternoon, in west central Missouri:

Western Missouri Medical Center in Warrensburg worked with the University of Central Missouri to provide COVID-19 Antibody testing for UCM employees. The testing took place from 3 to 6 p.m. Thursday in a parking lot on campus. I was tested.

From the CDC:

Using Antibody Tests for COVID-19 [Page last reviewed: May 28, 2020]

In general, a positive antibody test is presumed to mean a person has been infected with SARS-CoV-2, the virus that causes COVID-19, at some point in the past. It does not mean they are currently infected.

Antibodies start developing within 1 to 3 weeks after infection.

We currently don’t have enough information yet to say whether someone will definitely be immune and protected from reinfection if they have antibodies to the virus.

Healthcare providers who use antibody tests must know how the different tests work and use caution when interpreting test results:

If someone tests positive for SARS-CoV-2 antibodies but does not really have those specific antibodies, the result is a false positive. Similarly, if someone tests negative for SARS-CoV-2 antibodies but does really have those specific antibodies, the result is a false negative.

FDA has authorized antibody tests for this virus that have been submitted for their review. But these tests are not 100% accurate and some false positive results or false negative results may occur.

A higher percentage of positive results may be false positives when these tests are used in people who live or work in an area where very few people have had COVID-19.

People who receive positive results on an antibody test but don’t have symptoms of COVID-19 or have not been around someone who may have COVID-19 are not likely to have a current infection. They can continue with normal activities, including work, but still take steps to protect themselves and others.

People who receive positive results on an antibody test and who are currently or recently sick or have been around someone with COVID-19 should follow CDC recommendations on caring for themselves and protecting others, and when they can be around other people again.

Don’t

Antibody test results should not be used to determine if someone can return to work.

Antibody test results should not be used to group people together in settings such as schools, dormitories, and correctional facilities.

Also, from the CDC:

Interim Guidelines for COVID-19 Antibody Testing [Page last reviewed: May 23, 2020]

[….]
If a high positive predictive value cannot be assured with a single test, use an orthogonal testing algorithm. See Table 1 for examples of using one or two tests in populations with various prevalences of SARS-CoV-2 antibodies.
[….]

Currently in Johnson County, Missouri the number of confirmed cases (probably via PCR testing) is approximately 143.42 per 100,000 population. That’s 0.14% of the population.

The current CDC guidelines indicate a Positive Predictive Value (PPV) of 26.9% for one antibody test where 2% of the population has been infected. The CDC further states that the rate of false positives in antibody tests increases when used in populations with lower actual infection rates.
The PPV on a second antibody test after a first positive increases to 86.9%.

So, for those of us tested yesterday, a positive result is not going to mean squat, other than we’ll need to have a second antibody test.

The CDC also notes that there is no confirmation that the presence of antibodies indicates immunity.