Thousands of patients have been recalled for fresh blood tests amid confusion and delays following the amalgamation of two pathology services.
Almost 2,500 tests are being redone in Kent and Medway after complications meant many samples were either not tested or no longer viable for testing.
The health trusts have informed NHS England of the "serious incident" but don't believe any harm has been caused.
"High-priority" patients are being retested first.
The delays follow the merging of pathology services in north Kent in February in an attempt to "improve turnaround times".
The new system began in April, when "routine" blood tests from the Medway and Swale areas started to be tested at Darent Valley Hospital in Dartford.
It is understood that part of the hold-up was caused by differences in codes used for various tests by the Medway and Dartford and Gravesham trusts, and that there were also problems in getting the correct results to GPs.
The trusts have failed to respond to questions about the length of delays and nature of tests having to be redone.
But they said in a statement: "North Kent Pathology Service is aware that there have been some delays in processing a number of blood tests and we want to apologise to patients, GP colleagues and other healthcare professionals affected by this.
"We have been in touch with patients needing repeat tests directly, with those patients undergoing high-priority tests contacted first. Many patients have already been retested with their results put on the system for clinicians to review.
"A continuing clinical review has not identified any patient harm as a consequence of the delays."