How to deal with false negative and false positive COVID-19 results

Test results for COVID-19 may vary from one pathological laboratory to the other

HEALTH-CORONAVIRUS/CASES A woman reacts as a medical worker takes a swab from her to test for the coronavirus disease in Ahmedabad | Reuters

On Friday, US President Donald Trump and the first lady tested positive for the coronavirus, days after he received a negative test result and participated in the first presidential debate. 

Back in India, a 65-year-old woman, Usha Mutwa, staying in South Mumbai's Matunga, was told that she had tested positive for the novel coronavirus after her sample was collected by Suburan Diagnostics, a diagnostics centre and pathology lab in Mumbai. However, the family decided to get a second test done for reconfirmation. Accordingly, 24 hours later her sample was again collected by another pathological laboratory based in Dadar, SRL Diagnostics, which confirmed two days later that she tested negative for COVID-19. 

Two tests done in 24 hours showing diametrically opposing results led to confusion and panic among the family and deteriorated the mental and physical health of Mutwa who is now unsure if she has been indeed infected or not. The family chose to decide for themselves and went by the second report which had given a negative result. 

"Mummy did not have any symptoms. She only had a slight fever and that could even be due to typhoid, a physician had earlier explained to us. So we decided to go with the second report and anyway, my mother chose to quarantine herself for a fortnight at home," said Mutwa's 37-year-old son. 

This dilemma is increasingly being faced by people in cities across India, when test results for COVID-19 vary from one pathological laboratory to the other. It underscores how careful we need to be about treating any results as definitive when successful detection of the virus is not guaranteed. 

"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," says Dr Farah Ingale, director, internal medicine, Hiranandani hospital, Vashi. When a patient is tested for COVID-19 immediately after being exposed to the threat, the false negative is likely to occur; i.e. before the onset of known symptoms. Essentially, the false negative is similar to a test result that is wrong. This is because it indicates that the person is not infected when he actually is, or that he doesn’t have antibodies when he actually does, explains Dr. Ingale. 

Similarly, a false positive is also incorrect, as it indicates the person is infected when they are not, or that they have antibodies when they don’t. She says 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.  Also, at times, the swab samples may not contain the cells infected with the virus which might lead to a negative report. Poor testing and collection techniques may also lead to chances of the virus going undetected. Also, if the test is done at the very beginning of infection, then too, the results may show negative. However, in the case of Mutwa, the tests were carried out after the fifth day of her feeling unwell. 

Ideally, say experts, it is safe to treat oneself as infected if one has a high risk of getting exposed to the virus. The person may or may not have the symptoms. "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 should not be confused with genetic material of other viruses, hence 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," adds Dr Ingale. 

Patients have to be extra cautious as a number of factors play a role in the detection of the novel coronavirus in a patient's sample and these include the time and method of collection of the sample, viral load of the patient and the reagent used while testing. All these can change the results thereby leading to false negative or false positive reports.