How reliable are COVID tests?

Columbia Icefield (Dr. Noorali Bharwani)
Columbia Icefield (Dr. Noorali Bharwani)

COVID-19 pandemic took us by surprise and hit us hard, spreading from country to country and continent to continent. It has been with us for the last six months or so and shows no sign of stopping. In fact, we are warned to be ready for the second wave.

In the meantime, scientists all over the world are working round the clock to find an appropriated vaccine and antibiotic that would help. Most urgent focus has been to increase access to tests to diagnose people with current infections.

The possibility of COVID-19 should be considered primarily in patients with new-onset of fever and/or respiratory tract symptoms like cough and difficulty breathing. In some ways, COVID-19 is similar to influenza.

However, there are some key differences between COVID-19 and the flu. COVID-19 causes severe disease in a higher percentage of cases than seasonal influenza.

We need to do a laboratory (microbiology) test to establish COVID-19 diagnosis. Sensitivity of testing depends on the type and quality of the specimen obtained and the duration of illness at the time of testing.

In case of COVID-19, the virus that causes the disease – SARS-CoV-2 – is detected through a test known as a reverse-transcription polymerase chain reaction test, or RT-PCR. This is the most common type of test used in Canada.

A study of the accuracy of RT-PCR testing for COVID-19 found the test did not detect the virus on day one of infection and produced false-negative results 38 per cent of the time on day five and at least 20 per cent of the time after day eight. If patient has symptoms but the test is negative then test result should be ignored and patient should be kept in quarantine and tested again.

Types of testing

I have been curious to know how accurate are the tests done to diagnose COVID-19. Here is what I learnt.

Nucleic acid-based testing – for detecting nuclear acid specific to the virus. This is the preferred initial diagnostic test for COVID-19.

Scientifically speaking, this is a nucleic acid amplification testing (NAAT), most commonly with a reverse-transcription polymerase chain reaction (RT-PCR) assay to detect SARS-CoV-2 RNA from the upper respiratory tract.

This test is the gold standard used in Canada and abroad to diagnose active COVID-19 infection in patients with symptoms. There are very few false positive results. False negative results depend on timing of the sample in relation to the stage of the illness.

False negatives can occur up to 30 per cent of the time if we swab people who are asymptomatic and swab them too soon after they’ve been exposed to COVID-19.

False positives depend a lot on the machine itself, but also how much COVID-19 is in a town or community that is being assessed. It is hard to nail down a rate that applies to every machine that is used.

Test sensitivity depends on timing of sample collection. According to Centre for Disease Control and Prevention (CDC) proper collection of specimens is the most important step in the laboratory diagnosis of infectious diseases. A specimen that is not collected correctly may lead to false negative test results.

Serology-based testing – for detecting antibodies to the virus.

Serologic tests are blood tests that look for antibodies in the blood. They can involve a number of laboratory techniques. This test is not intended for use in clinical diagnosis. Serological tests do not detect the virus itself. Instead, they detect the antibodies produced in response to an infection.

Health Canada confirms that authorized COVID-19 tests are well supported by evidence indicating they will provide accurate and reliable results. That means very few false positive results. In Alberta, any person who wants to be tested can now be tested, even if they don’t have symptoms.

I hope this information helps. Take care. Be safe.

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