Testing for COVID-19

Updated on December 14th, 2020

Tests which may be performed for COVID-19 can be divided as follows:

Tests which specifically look for the virus.
Tests on the blood that look for evidence of antibodies which suggest current (IGM) or previous (IGG) infection.

There are several tests available which check for the COVID-19 virus:

PCR tests: these are the gold standard tests. They have the highest sensitivity and specificity.
Rapid tests (antigen tests): these tests are quicker and give results within 30 minutes but they have lower sensitivity and specificity and are therefore less accurate.

PCR Testing:

This is the method of testing used by the Hong Kong government. PCR tests can be performed on a variety of tissue samples. Most commonly we test deep throat saliva but deep nasal swabs (NP swabs) and even stool samples in children can be used.

Rapid Tests:

These tests are similar to pregnancy tests in that they give results very quickly. They are less accurate due to poorer (and less consistent) sensitivity and specificity. This is especially important when a condition is less frequent in a population (see below).

When the CHP announce provisional positive results they are NOT using rapid tests. The provisional results are PCR tests from designated and approved private laboratories in Hong Kong. All such tests are then confirmed in a government laboratory.

What is sensitivity and specificity?

The sensitivity of a test is a measure of how accurate the test is in measuring a true positive result. The higher this number the more likely that all the true positives have been identified. Specificity is a measure of how accurate the test is in measuring a true negative result. The higher this number the more likely that all the true negatives have been identified. Of course, things are never quite as simple in statistics and as the prevalence (number of cases in the population) falls. The predictive value of these tests also changes. Imagine a test with a 95% specificity looking for a condition that affected 5% of people. Although 95% seems like a high number in this case a false positive case (5%) is the same incidence as a true positive case (5% of population) which means that any positive test only has a 50% chance of being a true positive and a 50% chance of a being wrong result.

The prevalence of COVID-19 Hong Kong remains very low (a lot less than 5%) and for this reason the Hong Kong CHP use PCR testing at this time. At OT&P we only use PCR testing via a government approved laboratory.

Antibody Testing

There are also blood tests which measure antibodies against COVID-19 IGG. An IGG test becomes positive within the first few weeks of the illness. A number of reliable antibody tests are now available. The incidence of infection in Hong Kong is very low. Even in countries with widespread infection, the antibody studies suggest antibody levels which are below those needed for herd immunity. We do not yet fully understand how many infected people get antibodies, how effective they are in protection and how long they are likely to last. We are also beginning to understand the importance of the T-Cell response in immunity in COVID-19^[1]. There is no simple test which allows us to measure effective immunity. For this reason, the concept of an antibody test as an ‘immunity passport’ is still some way off. It is likely that PCR testing will be the mainstay of control for some time yet.

Test, trace and isolate in addition to social distancing measures has been and will remain the mainstay of management of COVID-19 until widespread vaccination is available. OT&P can arrange testing for patients with or without symptoms.

The Hong Kong Health department has also set up 14 testing centres in which anybody without symptoms can be tested for COVID-19. There is no charge for testing in the government centres except when taken for travel purposes. Further information about the government testing program can be found online^[2].

If you have any concerns or specific questions regarding this issue or if you wish to discuss the implications or logistics of testing for COVID-19, please arrange an appointment to discuss with your doctor.

References

1. Cookson, C., & Gross, A. (2020, July 17). T-cells: The missing link in coronavirus immunity?: Free to read. Retrieved December 14, 2020, from https://www.ft.com/content/5cf2ee49-df7a-4990-b337-860cf7737b2f

2. (n.d.). Retrieved December 14, 2020, from https://www.communitytest.gov.hk/en/faq/