Sat | Jun 12, 2021

Garth Rattray | Testing for COVID-19

Published:Monday | April 12, 2021 | 12:08 AM

Testing for COVID-19 is essential. In a perfect system, testing would be widespread, and free. That way, we could hunt down the virus, get ahead of it and block its transmission; instead, we are mostly chasing behind and documenting the disease. Hunting down the virus takes the kind of financial, technical, and logistical resources that we simply do not possess. So, we do what we can with what we have.


The antibody test: Our immune system produces antibodies to fight off infections. Antibodies take several weeks to develop and, in the case of COVID-19, they seem to last for probably about six months. A positive antibody (blood) test only tells you if the subject was previously exposed to COVID-19. It does not indicate current or even recent infection. Therefore, it should not be used to ascertain the infectivity status of anyone. The antibody test would be useful if there were a need to know if a previously infected individual still possessed antibodies to COVID-19. I suppose that it may come in handy to determine the effectiveness and/or longevity of the vaccination.

The antigen (rapid) test: This is by far the most popular COVID-19 test for several reasons; it is far less expensive and much faster than the PCR test for COVID-19. It can be done in regular medical practices and clinics, and there is no need for specialised technical training to conduct it. They utilise nasal swabs and a test kit. The results may be had in as little as 30 minutes.

The rapid antigen test is very useful but needs a fairly high viral count to be credible, and therefore, it also needs an appreciable lag time to be taken seriously. It is best done if the patient has symptoms. The test is pretty specific in that, if it comes back as positive, it most likely is accurate. However, even in sick patients, if it comes back as negative, it is not to be trusted and a PCR test must be performed. As always, because there may be false results, the history and clinical presentation are important.

The PCR test: This is the gold-standard test for COVID-19. It utilises nasal and/or throat swabs, but the testing equipment is very technical, complex, and very expensive. The results take many hours to a few days to be ready. It is highly sensitive and very specific. Because of these things, it is very believable and trusted.


The test can be wrong; in order to minimise errors, it should be properly timed. It takes about a week after exposure (give or take a day or two) for the virus to become detectable in our upper airways. Performing the PCR test a few days after exposure will probably yield a false, negative result. At the other end of the disease, the PCR test can detect viral fragments many days to many weeks after our bodies have cleared the virus and are no longer infected or infectious.

Because of this, there is no longer a need to have two negative COVID-19 PCR tests before being deemed virus-free. After 14 days (including three consecutive days with no symptoms attributable to COVID-19), patients may resume their normal activities. The exception is for moderate and severe cases.

My current disquiet with testing is the failure of many people to realise that the antigen (rapid) test is fallible, the average individual believes that a negative rapid test means that they are okay – this may not be the case. I am also peeved at the affordability of the PCR test and the difficulty associated with getting results from some major testing centres. When not contacted, many wrongly assume that all is well; they go along their merry way and could be spreading the disease, with devastating consequences. Learn about whatever test you are required to do, know its limitations, and always get the results before deciding your next move.

Garth A. Rattray is a medical doctor with a family practice. Email feedback to and