Some Welsh NHS staff with Covid-19 have been given wrong test results and were told they did not have coronavirus, BBC Wales has learned.
They are among a group of ten who have been given incorrect results - including eight from Aneurin Bevan Health Board and two from elsewhere.
It is not clear how many of the ten had Covid-19 and were told they did not, or vice versa.
The Gwent-based heath board said the staff were contacted "immediately".
It happened when a small number of test samples from a batch of 96 were attributed to the wrong patients.
Public Health Wales (PHW) said some clinicians who were positive for Covid-19 were told they were negative, and the other way around.
PHW said 10 out of 96 members of staff in a testing group were subject to "a recording error" which was picked up "within hours" by quality checking systems.
It is not clear where the other two individuals, which are not from Aneurin Bevan Health Board, are from. PHW confirmed that the testing was done in its labs and it was its error.
PHW said it contacted all the parties and health boards involved, and established no harm was caused.
The Aneurin Bevan Health Board area has seen the highest numbers of confirmed cases of Covid-19 in Wales.
Frank Atherton, the chief medical officer for Wales, has previously said that the large number of tests carried out there, along with its proximity to London, help to explain why it has become a hotspot for the virus.
A spokesman for the health board said: "Since the 14th of March we have undertaken circa 1,600 staff tests.
"As part of our checking process, we identified a local transcribing issue with eight test results that led to us giving staff members wrong results."
The spokesman said the staff were contacted "immediately", and the health board has undertaken a "detailed review" of all staff tests and "taken action to remove any further risk of transcribing errors".
BBC Wales has asked whether any of the staff members given false negatives attended work before being told they were, in fact, positive.
In a statement PHW said it followed up the cases "to establish if any harm had occurred due to the incorrect information being communicated to individuals."
"It was established that no harm had occurred," it said.
"We continue to have complete confidence in the testing process, and the laboratory staff carrying out the testing procedures," PHW added.