by Dr. Nell Thomas
COVID-19 diagnostic testing is done to find out if you are, or have been, infected with SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19). There are three main testing technologies used in Canada: PCR, Antigen, Antibody.
The PCR (polymerase chain reaction) test is the gold standard, with detection close to 100 per cent when processed at one of the 11 Public Health Ontario Labs. It detects the virus’ genetic material (RNA) using a swab to collect fluid from the nose, nasopharynx, throat, or saliva you spit into a tube. Sputum from lungs may also be used. PCR tests are very accurate when properly performed but results may vary depending on collection technique, and the processing lab. Most take one to three days for results, with some in 15 minutes.
Antigen tests are less expensive and faster (results in 15-30 minutes) but less accurate. These detect viral antigens (“toxin”). Performed on nasopharyngeal or nasal swab specimens, they are not as sensitive as PCR, and earlier in the illness they miss about 20 per cent. They often need confirmatory testing with PCR.
PCR and Antigen tests detect current disease. Antibody tests identify prior infection (or vaccination). Antibodies usually start developing within one to three weeks after infection. They are not used to test active infection and cannot be used to indicate when a person stops being contagious. A prior infection with a different coronavirus can cause false positives.
Rapid (Point of Care) tests provide quick, cost-effective diagnosis, critical in areas with significant disease. India, with less than 10 per cent of its population vaccinated, developed a test with results in four minutes, and 90 per cent accuracy. It is called RAPID 1.0 (Real-time Accurate Portable Impedimetric Detection prototype 1.0). An electrode detects the SARS-CoV-2 in saliva samples or nasal swabs. Results from the samples are read from a smartphone or a laptop. RAPID is 90 percent accurate for saliva and 87.1 percent accurate for nasal swabs.
In the USA, the FDA granted emergency use authorization for certain at-home COVID-19 test kits. Most require a doctor’s prescription. You collect your own sample of nasal fluid or saliva at home and send it to a lab. One COVID-19 test provides fast results at home, no lab.
At this time, in Canada, we haven’t authorized at-home test kits. However, Ottawa and Queen’s Park teamed up to make rapid-testing kits available to workplaces. In Simcoe, Ontario the Chamber of Commerce has thousands of kits and is distributing them to workplaces on a first-come, first-served basis. (Haliburton will also get the kits.)
Zimbabwe is using antigen rapid tests – about 4,000 a day. They have been a game changer. Used in rural areas, results are received in 20–30 minutes, compared to one week when results through PCR testing were sent back to distant laboratories.
Singapore has provisionally approved a COVID-19 breath test that is greater than 90 per cent accurate with results in less than one minute. Developed by Breathonix, it will be administered by trained personnel, but does not require medically-trained staff or laboratory processing.
And now, the dogs. In under one second, sniffer dogs can detect COVID-19 with 82-94 per cent accuracy. In a collaborative effort in England, scientists completed Phase 1 of a trial showing dogs’ ability to identify COVID-19 on samples of clothing and masks. In six to eight weeks of training, dogs learned to detect the virus in asymptomatic people, and those with low level of illness.
Accurate as they are, dogs will never replace PCR tests. At airports, dog-identified passengers require a confirmatory PCR test, quarantining while awaiting results.
That’s better than requiring everyone to quarantine and undergo PCR tests. Finland, France and Lebanon are conducting trials with sniffer dogs as well.