Eight NHS Wales staff were given the wrong results after being tested for coronavirus.
They work for the Anuerin Bevan University Health Board, which is the area with the highest number of confirmed coronavirus cases in Wales.
Public Health Wales said "the error was identified within a few hours" and "no harm" came to the individuals involved.
It has not been disclosed how many had Covid-19 and were told they had tested negative, or vice versa.
10 samples tested were assigned to the wrong people, eight of whom were staff from the Aneurin Bevan Health Board area.
A spokesperson for Public Health Wales said checking processes meant "the error was identified within a few hours."
The samples were then retested so the correct results could be given to the right people.
The quality checking processes ensured that the error was identified within a few hours, and the samples were retested to obtain the correct results which were communicated to the clinicians who had requested the tests.
They added that since March 14, they have carried out around 1,600 coronavirus tests on healthcare staff.
Aneurin Bevan has 1160 confirmed cases - the most of any health board in Wales. Cardiff and Vale University Health Board have the second highest with 943.
Public Health Wales said it has conducted more than 15,000 tests for coronavirus on more than 13,000 individuals since the start of the outbreak.
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