Wales in the UK is reporting hundreds of botched up prostate cancer tests. Those given all clear might have to go through biopsy again.
The "discrepancies in the reporting of specimens" came to light during a series of quality checks carried out by the trust.
Consequently more than 500 men tested for prostate cancer have been told their results must be rechecked.
Carmarthenshire NHS Trust, in west Wales, has sent out letters to 528 patients informing them a raft of complex rechecks must be carried out.
Trust bosses have brought in an independent agency to carry out the test reviews which could take up to seven weeks to be completed.
The outcome could mean that some men who had been given the all clear after prostate biopsies will be called back.
But the trust insists that only a small minority of patients will be directly affected, with the majority of checks confirming original results.
"All patients whose cases are being reviewed have been sent a letter by Carmarthenshire NHS Trust in order to keep them fully informed," a trust spokesman said yesterday.
"In those cases where inaccuracies may have occurred, patients will be contacted directly by a hospital clinician or GP as soon as possible and their care will be reviewed.
"All other patients, whose specimens have been checked but confirmed as correct will also be sent follow-up letters in order to reassure them."
He added: "Current prostate patients are being reassured that additional stringent procedures have been introduced for both positive and negative samples.
"It is stressed that any patients involved will have received a letter and will be kept fully informed of progress as the checks are carried out."
Carmarthenshire NHS Trust's medical director Dr Peter Thomas said: "The care and well- being of our patients is an absolute priority for the trust.
"Therefore, following a clinical audit that identified discrepancies in the reporting of prostate biopsies, we have taken steps to review some 500 cases as a precaution.
"This does not include blood or urine samples and I would like to stress that any patient involved would have been contacted directly by the trust by letter.
"Reviews are currently being undertaken and patients will be informed of the results as soon as possible."
The trust's chief executive Paul Barnett said: "The trust would like to publicly apologise for any distress this may cause the patients involved and to reassure them that the necessary steps are being taken to ensure that any errors are detected and dealt with as soon as possible."