Q) I hear a lot about testing for Covid-19 and there seems to be a number of different ones out there. What are the differences and which one is best?
A) Many experts feel that the two pillars that will allow us to eventually end this pandemic are effective and easily accessible diagnostic tests and vaccines. We have spent a few weeks discussing the vaccines so, I thought today, we would start to discuss the various testing options. While there are a number of tests out there, they fall into two broad categories.
There are diagnostic tests which can show if you have an active infection right now and antibody tests which provide evidence that you have previously been infected by Covid-19. Diagnostic tests are performed far more frequently and are the key to identifying affected individuals and getting them into quarantine while public health begins the task of contact tracing.
The tests that are done up at our local hospital and other sites manned by Ontario medical professionals are ones that use polymerase chain reaction (PCR) technology to identify the virus. They are also known as molecular tests.
PCR tests are designed to look for very specific genetic information of the virus (its RNA for those of us who still faintly recall our biology classes). These tests are highly sensitive. They can detect the virus in your body very early on, even before you start to exhibit symptoms. This test will also detect the asymptomatic individuals who may have the virus but do not present with any symptoms. It is felt that they can accurately detect the virus within 5-7 days of exposure. Recall that this coronavirus has a 14 day incubation period which means that when you first get infected, you are very unlikely to realize it. Thus these tests are an effective way to stop the spread of the disease when used on a large scale basis.
They can also detect the virus for a period of time after a person is no longer actively sick or shedding (spreading) the virus. PCR tests are relatively labour intensive and therefore take longer to obtain results and cost more when compared to other tests. The process starts with a swab in order to obtain a sample of fluid. The preferred method is the dreaded nasopharyngeal swab in which the sample is taken from the very back of your nose. Though this procedure isn’t regarded as being painful, it is best described as unpleasant or uncomfortable and can make your eyes water (it might bring back memories of that classic scene from Schwarzenegger’s bid for an Oscar in Total Recall). Experts believe this process allows for the most accurate results due to the attainment of a better specimen.
For those that cannot tolerate this method, a series of samples can be taken from the back of the throat and both nostrils. Once the sample is obtained, it is shipped to a lab where a technician will use chemicals to extract the genetic material from the extraneous fluid. The extracted material is then placed in a test tube with new chemicals and is put in a machine with many other collected specimens at once. The machine enables a “PCR” reaction to occur in which millions of copies of the virus’ DNA are produced in a process known as amplification. This allows even a small amount of virus to be detected although, once again, you can be infected but have so little of the virus present that it escapes detection. The results are then sent to the health care practitioner who ordered the test and are posted on the ministry’s website. All positive tests are also reported to the local public health unit who will then contact the patient and check on their well-being as well as provide them with instructions as to how to self-isolate. This is also when the process of contact-tracing starts.
How long does all of this take? Well in theory it could take only a few hours to yield results but tests done by our local health care team have a typical turn around period of between 2 to 3 days. This can vary from other regions in Ontario. Accuracy is pretty reliable but not perfect. The rate of what we term false positives (i.e. when the test says you have Covid-19 but you actually do not) should be pretty close to zero. Most false positives are a result of how the lab performed the test (i.e. was the sample contaminated?) or how they recorded the results.
False negatives are more common. A false negative result is when the test says you don’t have the virus when you actually do. The rate depends on a number of factors but one study placed it at 20% when the test was performed 5 days after symptoms began. The rate of false negatives does increase if the test is performed even earlier and decline if it is delayed by a few days. This is one of the reasons that the government is having Canadians who have been outside of the country take a PCR test immediately upon returning as well as a second test towards the end of their 14 day quarantine.
In the next week or two, we will look at the other type of diagnostic test which is commonly referred to as rapid testing as well the antibody detection tests we mentioned at the outset. Stay tuned as well to learn about the new and booming industry that allows individuals to order a test themselves through a private lab. For more information about this or any other health related questions, contact your pharmacist.