Covid 19 Tests: What we can and can’t say.

I am a data driven person by training, education and personality, but data must be very carefully examined – ‘granularly’ as Dr. Birx would put it.  Simple interpretations of data can provide some very wrong direction.

Much of the discussion around the management of Covid 19 has been around the concept of testing.  However, the details of testing and what you can and can not say about the results of testing are almost never discussed.  As of April 19, the US has conducted 3.7 million tests.  The country with the next most test is Germany with 1.7 million and then Italy with 1.3.

Are there different tests?  Who needs to get tested?  Does everyone need to get tested?  These and other questions will be addressed here.

First, there are 2 general kinds of testing, molecular and serological.

Molecular tests were the first type of Covid 19 test available from the CDC and WHO.  At this time 22 companies have received FDA authorization of distribute these tests and additional 50 companies are preparing their applications for approval.  The Abbott 5 minute test is a molecular tests.

In a molecular test, a non blood sample such as a nasal swap or saliva is collected.  The molecular test detects the presence of genetic material from the virus.  In this test, the genetic material from the swab is copied to make large enough quantities for use and the compare the known genetic sequence of the corona virus.  If you want to impress you friends, this process is called reverse transcription polymerase chain reaction of RT-PCR.  Depending on whose product you use, the time for analysis in the machine is 5 minutes to 48 hours.  Note that often the rate determining step is getting the sample to the lab, often the lab is not the hold up.

What does the molecular test tell you.  A positive test means that is possible that an active Covid 19 infection has occurred.  However, a positive test does NOT rule out bacterial infections of infections by other similar viruses (eg influenza)

A negative samples means that there was no Covid 19 infection found.  However, if you were tested soon after you were infected, they may not be enough virus to be detected.

The guideline is a positive test is indicative of Covid 19, especially if you have a hard time breathing and fever, but there is still a chance that you do NOT have the virus.

If you test negative, you have either been too recently infected for detection or you don’t have the virus.  The only way to be sure is to be tested again in a couple of days (if you are still symptomatic)

Serology Tests

These are the antibody tests everyone is talking about.  The human body is an amazing thing.  When the body detected a new infection, like Covid 19, it sets about making a protein specifically to combat the virus.  This takes awhile but eventually the body develops a protein which will kill or neutralize the virus.  The proteins it makes are termed immunoglobulin (Ig).  The presence of immunoglobulin M (IgM) in the blood indicates a recent infection.  The presence of immunoglobulin G (IgG) indicates that the infection has been around for awhile.  The test used to detect the presence of these proteins is called ELISA (enzyme linked immunological assay).

It typically takes less than 20 minutes to do an ELISA.  However, it can the body several days to make enough IgM to be detectable, so if you get tested too soon after infection you test negative.

If you test positive for IgG, then you probably had the disease and in the best of worlds, you are immune.  However, it is critical know that (at this point) we do not know how long the immunity lasts or even if it’s possible to get reinfected even though you have the antibody. If you test negative, then you probably have not had the virus, but it doesn’t mean you won’t get it tomorrow.

Key points:

Molecular tests are available but will only tell you if you are likely to currently have the disease.  You could get it tomorrow.  This means that there is little reason for asymptomatic people to have the tests.  It only means you are infected the day of the test.

Even if the molecular tests is positive, it is not 100% assured you have covid 19.  There are some number of false positives and the positive result may be from other strains of related flu or bacteria.

The antibody tests must be run several days after you are infected, otherwise it will be negative.  It is not know what level of immunity a positive tests means.

A key specification that is never discussed in the news is the specificigy and sensitivity of each test.  That is each test a different accuracy rate of the number of false positives and false negatives.  The much vaunted Korean test has over a 20% error rate for instance.

Final Points

  1. It is not likely necessary or desirable that everyone get tested. Especially if you are asymptomatic.  Going in to get this test while you are asymptomatic gives little helpful information as far as treatment goes and does not tell you anything about tomorrow if you are negative.
  2. Antibody testing would be important from an overall disease understanding point of view but is unclear how useful it is for treatment.
  3. The common sense approach is still the best drving force. If you feel sick, especially with respiratory issues and fever then either stay home (if the symptoms are mild) or call your physician and they will give you instructions in how to come in and get help…try not to just want walk into a drs office of hospital (exposing others) unannounced.


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