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Mild COVID-19 symptoms might not result in positive antibody test,

30/07/2020

Oxford researchers have found that COVID-19 antibody tests might not give a positive result if people have only had mild symptoms, like loss of taste and smell.

Microbiology and data experts from the Oxford University Hospitals (OUH) NHS Foundation Trust and the University of Oxford made the initial findings, published as a pre-print, as part of their comprehensive testing programme for OUH staff.

The researchers previously found that among OUH staff an isolated loss of smell or taste was strongly predictive of COVID-19 infection, as defined by a positive swab or antibody result.

Antibody results are however defined by a threshold set against a continuous scale with only readouts above the threshold defined as 'positive'. The researchers have now looked at readouts below the threshold and have found an increasing proportion of staff reporting loss of smell or taste as the negative readouts increase and approach the threshold for positivity.

Of those who had a 'low antibody' antibody test, approximately four percent reported loss of smell or taste, compared to 13-15 percent with a 'high negative' test and around 47 percent with a positive antibody test. 

Dr Tim Walker, one of the authors of the paper, said:

"Our dilemma is that some people with a mild form of the disease can fall into an 'equivocal' zone that is currently classed as 'negative' result. Lowering the threshold to reclassify some false negatives as 'positives' will inevitably mean that some true negatives will then wrongly be categorised as positive.

"When we adjust for the likely proportion of staff reporting a loss of taste or smell due to reasons other than COVID-19, it suggests that the sensitivity of the antibody tests are 9-11 percent lower than had previously been reported.

"It means that we need to consider those individuals with the full spectrum of mild to severe infection when we evaluate these tests. We could also consider reporting 'high negative' antibody results as 'equivocal'.

"What the test is being used for should of course influence any such decision."

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