False negatives and false positives in COVID-19 cases; find out what they are

'While both false negatives and false positives are undesirable, false negatives run the risk of increasing community transmission,' says Dr Farah Ingale, of Hiranandani Hospital, Vashi

A person can be declared infected or COVID-free upon being tested. While the test results decide the next course of action, sometimes they may mislead them into believing they are safe. Dr Farah Ingale, Director-Internal Medicine, Hiranandani Hospital, Vashi — a Fortis Network Hospital, says when a patient is tested for the COVID-19 virus immediately after being exposed to the threat, that is when the false negative is likely to occur — that is before the onset of known symptoms. Essentially, the false negative is similar to a test result that is wrong.

“This is because it indicates the person is not infected when they actually are, or that they don’t have antibodies when they actually do. Similarly, a false positive is a test result that is incorrect, as it indicates the person is infected when they are not or that they have antibodies when they don’t,” she says.

Dr Ingale suggests that patients who are at a high-risk to virus exposure, should be treated as if they have been infected, especially if the symptoms are consistent with COVID-19. “This means communicating with patients about the tests’ shortcomings. If a swab misses collecting cells infected with the virus, or if virus levels are low earlier on during the infection, some RT-PCR tests may produce negative results. Since the antigen tests return relatively rapid results, they have been widely used among high-risk populations such as nursing home residents, hospitalised patients, and healthcare workers. Previous studies have shown or suggested false negatives in these populations.”

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She goes on to say that researchers projected that those patients tested with SARS-CoV-2 within four days after infection were 67 per cent more likely to test negative, even if they did have the virus. “When the average patient began displaying symptoms of the virus, the test performed best eight days after infection (on average, three days after symptom onset), had a false negative rate. The sooner people are accurately tested and isolated from others, the better we can control the spread of the virus, researchers say.

“While both false negatives and false positives are undesirable, false negatives run the risk of increasing community transmission, should individuals erroneously believe they’re not infectious and fail to take necessary precautions. This can occur whether people have no symptoms, or have symptoms but assume they’re due to something other than COVID-19,” the doctor warns.

When to treat negative as negative?

Any patient with symptoms in a hotspot, who tests negative may be reasonably assumed to have the virus, while an asymptomatic patient in an area of low transmission who tests negative can probably take comfort in that negative result, says Dr Ingale.

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Some recent studies have highlighted a significant rate of false negatives — particularly if the test is administered soon after exposure. “In these cases, viral material may not be captured through testing as the virus content in the person is not enough to be picked up. It’s also possible that poor testing technique can miss out on any virus that is present.”

Diagnostic test

The PCR test for COVID-19 works by detecting genetic material from SARS-CoV-2, the virus that causes COVID-19. Genetic material from SARS-CoV-2 is not to be confused with genetic material of other viruses, because the diagnostic test for COVID-19 should be specific. “If a person is tested positive for COVID-19, one can be sure that he/she has been infected. The antigen test for COVID-19 is also accurate which seldom gives a false positive,” Dr Ingale concludes.

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