A fifth of people with negative coronavirus tests might have the virus, the chair of the Commons health committee has said.
Jeremy Hunt said evidence suggested 20% of negative results could be false.
He was questioning Dido Harding, head of the Test and Trace service, over its performance since the launch last week.
It followed Boris Johnson telling MPs at Prime Minister’s Questions tests would be processed within 24 hours by the end of June.
But, questioned by Mr Hunt, the PM said the 24-hour target would need to take into account “insuperable problems” such as postal delays.
“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.”
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.
During her evidence session at the select committee, Baroness Harding said estimates of the proportion of tests that gave false negative results ranged between “two and 20-odd per cent.
She said the issue of why those with a negative result were not retested was “a medical/science question”, adding “my job to take Sage and chief medical officers’ guidance”.
The Hospital Consultants and Specialists Association (HCSA), which represents hospital doctors, had earlier called for NHS staff to be tested more than once to confirm a negative result.
Dr Claudia Paoloni, the union’s president, says relying on a single negative result risked “infecting patients and staff”.
GP, and diagnostic tests researcher, Dr Jessica Watson told BBC Radio 4’s More or Less programme the “best guess” was about 70 in every 100 people who have coronavirus would be picked up by the testing programme.
Factors affecting the outcome include at what point in their illness someone is tested, how good a sample is taken, along with any problems with processing tests, she said.
Public Health England is expected to publish an evaluation of its tests, used for hospital patients and some staff, this week.
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