COVID Testing FAQ's
How is COVID-19 diagnosed in PEI?
Our provincial laboratory uses validated PCR (polymerase chain reaction) tests for the diagnosis of COVID-19. A validated PCR test is considered the gold standard for the diagnosis of COVID-19. The PCR test techniques used in our provincial laboratory have been validated according to national standards and are under continuous quality assurance.
What is PCR?
Polymerase chain reaction is a method of testing which repeatedly copies a portion of genetic material by running multiple ‘cycles’ of a copying process. Through these cycles of the copying process, a tiny amount of genetic material is made bigger so that it can be checked for the presence of a genetic marker that is unique to a pathogen, such as SARS-CoV-2 (the virus that causes COVID-19).
What is a positive PCR result?
A positive PCR result occurs when the genetic marker of the virus is detected in a sample after the sample has gone through cycles of the copying process.
The PCR tests that we use are highly specific for SARS-CoV-2, the virus that causes COVID-19. This means that the test does not react to other viruses, including other coronaviruses. If the test comes back positive, we can be confident that it has correctly detected the genetic material for the SARS-CoV-2 virus. False positive results are extremely rare.
What is a cycle threshold?
A cycle threshold refers to the number of copying cycles it takes for the genetic material of a virus to be detected by a PCR test. The higher the number of cycles, the smaller the amount of virus in the sample.
The amount of virus in a sample, and thus the cycle threshold value, is dependent on numerous factors. In fact, different samples from the same person may result in different cycle threshold values. Individuals with a recent exposure to COVID-19 may still be incubating virus and not shedding very much virus. A poorly collected sample can also result in low virus levels. The type of sample is also important - nasopharyngeal swabs (the very deep swab!) usually collects larger quantities of virus than a just nose swab. Finally, even after recovery and the body has successfully fought off the infection, small amounts of virus, or pieces of virus, may still be present.
All this to say that, the interpretation of cycle threshold is complex. Cycle threshold values are always interpreted in relation to clinical and other factors, such as patient history, clinical presentation, the quality of the diagnostic sample, the sample type, and the PCR technology being used.
Any test suspected of being a false positive is investigated further (e.g., repeat testing using a nasopharyngeal swab, and/or repeat testing within 24 to 48 hours). Similarly, in situations where the clinical index of suspicion is high, (e.g., in a known close contact of a confirmed case), a negative test does not necessarily rule out disease. Individuals with a recent exposure may still be incubating virus and not shedding enough virus to be detected by PCR.
What is rapid point of care testing?
The rapid point of care test used in PEI is the Abbott ID Now. The Abbott ID Now is a molecular test that uses similar technology to the lab-based PCR test. The benefits of the Abbott ID Now test is that it can be used outside of the lab setting and results are available within 15 to 20 minutes.
In PEI, the Abbott ID Now is being used as a screening test and all positive results on the Abbott ID Now are considered preliminary. Individuals with a preliminary positive result are sent for confirmatory testing using a nasopharyngeal swab and a lab-based PCR test.
What is a rapid antigen test?
Similar to the Abbott ID Now, rapid antigen tests can be used to screen individuals for COVID-19. However, rapid antigen tests have a lower sensitivity and accuracy than molecular tests (e.g., PCR and Abbott ID Now) because they do not use the copying technology (i.e., the amplification step) that is used by molecular tests.
Rapid antigen tests perform best when COVID-19 infection rates are high. We have not used rapid antigen tests for COVID-19 screening in PEI.
Are the COVID-19 tests used in PEI able to diagnose someone who is infected with a variant of concern?
Yes. The variants of concern identified to date do not affect diagnosis though any of the tests used in PEI. All specimens that are positive for COVID-19 in PEI are sent to the National Microbiology Laboratory in Winnipeg for genetic sequencing and identification of variants of concern. Data on variants of concern detected in PEI can be found here.