Asymptomatic CoVID testing is being offered in Nova Scotia – What is it and how does it work?

Asymptomatic CoVID testing is being offered in Nova Scotia – What is it and how does it work?

By Dr. John Ross MD FRCPC,  | December 4, 2020

After almost a year of COVID-19, we have learned that 1) The virus is spread mostly by breathing ‘aerosol’ 2) Masks therefore make a big difference in decreasing spread 3) Everyone is different if they get an infection – some very sick, some a little sick, while others have no symptoms and are unaware of their infection.

There are two main types of tests looking for active infection: 1) The ‘gold standard’ PCR that tests for specific genetic material 2) Antigen or LOOP tests that test for other parts of the virus. There are also antibody tests, but they look for PAST infection, in the last few weeks or months, and cannot identify an acute infection reliably.

The PCR test is conducted at the NSH laboratory, under high quality conditions, and has both a very high specificity (a positive test is definitely the CoVID virus) and sensitivity (a negative test is quite likely negative.) Different language is used there because false negatives are sometimes possible (see below.)

The antigen test (in NS it is the Abbott Panbio) is rapid, around 15 minutes after the swab is taken. It also has a high specificity (likely true positive when virus is out in the community) but a lower sensitivity – it has a higher false negative rate. So it is not a ‘great’ rule-out test. But because it is rapid and cheap, more people can be easily tested, and although we may miss a few people, we will also find a bunch of people who do not know they have the virus and could be spreading it. So for a large group of people, it is a good test to use.

The antigen test is not lab quality and does not produce a lab certified result. It is not acceptable for travel to other countries or workers going to remote areas, and is not good enough for close known CoVID-19 positive contact tracing. The PCR test, while it takes longer and is more expensive, provides more information and is higher quality.

Test timing can also be important. If someone comes into close contact with a known CoVID-19 positive person, it will take a few days of incubation for that contact to become test positive if they were infected. Testing too early can produce a false negative result. There are other reasons false negatives occur. So an immediate test is not useful other than to rule out a previous, unknown infection. People exposed should self-quarantine X 14 days. In NS all close contacts are managed by NS Public Health.