The HSE has said that some patients were wrongly told they had tested negative for Covid-19.
In a statement released this afternoon, it said the error occurred because some lab tests came back with indeterminate results.
"Unfortunately these indeterminate cases were initially reported as not detected and consequently were notified to people as such in line with the results communication process," said the statement.
This meant the patients were informed that they had tested negative for the virus.
The HSE said less than 100 patients were affected.
It said that in ordinary circumstances the patients would have been retested but this did not happen because it was not possible to request another sample in the required time-frame.
The National Virus Reference Laboratory reviewed the detailed test data from the original test results and retested the original samples. As a result it was decided that some were, in fact, "weakly positive".
The HSE said a team of public health clinicians has contacted or attempted to contact each of the people involved to advise them of the correct outcome of their test, and to provide the appropriate public health advice and support to them.
The HSE said it is satisfied that no other patient has been impacted by this error.