Explained: Why ‘false negative’ coronavirus tests are a concern

Written by Anuradha Mascarenhas
| Pune |

Updated: April 13, 2020 9:56:47 am

The test can be negative if the sample is not obtained or processed correctly or even obtained too early, said a scientist at the National Institute of Virology. (File Photo)

There has been concern about the manner in which some COVID-19 patients have apparently relapsed. Only days after testing negative, they have been confirmed positive a second time. In Pune, a woman in her sixties tested negative, then fell critically ill with the infection three or four days later, and subsequently died.

Are these fresh infections? Doctors do not rule out the possibility that these patients had not rid themselves of the virus in the first place, but the virus didn’t show up in the tests. This is called “false negative”.

Coronavirus (COVID-19) ‘false negative’ tests: Why it happens

No lab test is 100% accurate, said Dr Marc-Alainn Widdowson, Director at the Institute of Tropical Medicine, Antwerp, and formerly with US Centers for Disease Control and Prevention. He has researched the subject of false negative tests.

“The tests based on detection of genetic material are very sensitive, but yes sometimes are negative,” Dr Widdowson told The Indian Express. “It can be because the swab was not taken right or the test was run badly, or sometimes simply because the virus may shed in different amounts, and was not there in the nose at that time. If the infection is in the lung, then a nose swab may not detect it. To be confirmed negative after being positive… you normally need two negative swabs 24 hours apart to be sure.

In 2003, Dr Widdowson had done a study on SARS that showed respiratory swabs can be negative, but faeces positive; so the virus can exist in the body even if not in the nose at a given time.

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Dr Harlan M Krumholz, professor of medicine at Yale University, wrote in an opinion piece in The New York Times that an initial swab sample may not always collect enough genetic material to provide an accurate test. This problem may arise more often in patients who do not show many symptoms at the time of their test.

This is an indication that lessons are still being learnt, according to Dr Subhash Salunkhe, chairman of the Maharashtra state technical committee on preventing communicable diseases.

No lab test is 100% accurate, says Dr Marc-Alainn Widdowson. (File Photo)

Need for caution

With limited public data on false negative rate in the clinical setting, we must regard each test result that is negative in a guarded fashion, Dr Salunkhe said.

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The test can be negative if the sample is not obtained or processed correctly or even obtained too early, said a scientist at the National Institute of Virology. “All tests suffer from false positive and false negative errors. We are constantly battling with positive and negative predictive values,” said epidemiologist Prof Jayaprakash Muliyil, who also observed that finding coronavirus on a person who died does not mean that the person died of the virus.

False security

In a special article in Mayo Clinic Proceedings, experts have flagged another concern — the risk that over-reliance on COVID-19 testing can pose to clinical and public health decisions. “A negative test often does not mean the person does not have the disease, and test results need to be considered in the context of patient characteristics and exposure,” said Dr Priya Sampathkumar, an infectious diseases specialist at Mayo Clinic.

The authors stress it’s important for public health officials to stick to principles of evidence-based reasoning regarding diagnostic test results and false-negatives. “For truly low-risk individuals, negative test results may be sufficiently reassuring. For higher-risk individuals, even those without symptoms, the risk of false-negative test results requires additional measures to protect against the spread of disease, such as extended self-isolation” said Dr Colin West , a Mayo Clinic physician.

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