Health Page: What a COVID-19 test entails


    Justin Rother, PharmD candidate, BSc.

    How do I know if I should get tested?

    Ontario residents who have been exposed to COVID-19 or are experiencing symptoms of the virus should take the province’s online self-assessment test to determine their eligibility and if they should get tested. Patients should then call Telehealth Ontario after completing the assessment for follow up on what to do next.

    What tests are available for COVID-19?

    Two kinds of tests are currently available for COVID-19, viral tests, and antibody-based tests. A viral test is used to determine if you have a current infection while the antibody test will tell you if you have previously been infected. As it can take 1-3 weeks after infection to make antibodies, the antibody test may not show a current infection. While antibodies may protect individuals from getting infected again for other infections, it is yet to be determined if having antibodies to COVID-19 can provide immunity to the virus. We will focus on the viral tests in the discussion below as they are currently in use in Ontario.

    How are samples for the test collected?

    Current testing methods in Ontario include collecting samples via nasal swab, throat swab and nasopharyngeal swab. A swab is a brush-like device that is used to collect a patient’s specimen. The swabs used for the nasopharyngeal method are thin and flexible while larger, sturdier swabs are used to collect specimens from the nasal and throat. While different manufactures require different methods of sampling for the tests, the nasopharyngeal swab is the preferred method of obtaining the sample and most used due to the increased accuracy of the test.

    What to expect from the nasopharyngeal test?

    The patient will begin with blowing their nose and then tilting their head back. This allows for easy access to the nasal passageways for the health care professional. Next, a long flexible swab is inserted into the nostril of the patient which allows the sample to be collected. The swab goes quite deep, all the way back to the upper part of the throat behind the nose. This is called the nasopharynx. The swab must remain in the nostril for a few seconds to ensure enough material is collected, and as such, the test may be quite uncomfortable for the patient.

    How does the viral testing work?

    Current testing in Canada uses Polymerase chain reactions (PCR) assays. PCR works by amplifying regions in the viral genome such as the viral envelope or the nucleocapsid. The viral genome contains RNA genes which are converted into DNA and then amplified through the polymerase chain reaction to be analyzed. The advantages of PCR include automation, reliable instrumentation and higher throughput as well as being extremely accurate.

    How accurate is the test?

    The sensitivity of a clinical test refers to the ability of the test to correctly identify those patients with the disease. Studies have shown that the PCR test has a sensitivity of 70 % to 90 %. That means that it can correctly identify about 80 % of patients who truly have the disease. Approximately, 20 % of those tested will be false negatives, in other words, despite having the disease, the test will identify them as being negative. As such, when a patient tests negative but is displaying symptoms, it is important to not be over reliant on the negative test result and for them to stay at home until they feel better. On the other hand, false positives are extremely rare for the COVID-10 PCR test. As such, if the test result is positive, this indicates that the patient almost certainly has COVID-19.