The head of the Test and Trace programme has been challenged over the risks of false negative coronavirus results by a committee of MPs.
Dido Harding was asked at the health select committee why people are not repeatedly tested to ensure they do not have coronavirus.
MPs heard 20% of positive cases may be missed by the test.
Earlier Boris Johnson told the Commons tests would be processed within 24 hours by the end of June.
Questioned by Mr Hunt, the PM said the 24-hour target would need to take into account “insuperable problems” such as postal delays.
He said “I can undertake to him now to get all tests turned around in 24 hours by the end of June, except for difficulties with postal tests or insuperable problems like that.”
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The tests involve a nose and throat swab which is then used to look for the virus’s genetic material. It detects active coronavirus infection, so cannot say if someone has recovered from Covid-19.
Testing positive should allow contacts to be traced, and determine whether NHS staff can go back to work.
But the swab tests carried out can deliver false negative results – suggesting someone does not have coronavirus when they are actually positive.
That could be because of the quality of the swab, the timing of the test or issues in the lab.