More than 500 men tested for prostate cancer have been told their results must be rechecked after the discovery of potentially fatal testing errors.
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.
The "discrepancies in the reporting of specimens" came to light during a series of quality checks carried out by the trust.
"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.
"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."
An information line has been set up for anyone who may have concerns or queries and can be contacted between the hours of 9am and 5pm Monday to Friday on 01267 227717/ 227724.
