A small number of people tested for syphilis in England and Scotland have been given inaccurate results, the Health Protection Agency (HPA) has said.
Experts are investigating test kits used on the NHS to diagnose the bacterial infection, which is passed on through sex.
So far, more than 93% of people affected by the kits have been traced, with 75 patients found to have inaccurate results.
Around half were diagnosed with syphilis when they did not have it, while the remainder were identified as being in the later stages of the disease when they were in the early stages.
The kit, which has been withdrawn, was used between November 2010 and September 2011.
If diagnosed early, syphilis can be easily treated with antibiotics, usually penicillin. But if left untreated, it can progress to a more dangerous form of the disease which can eventually cause stroke, paralysis, blindness and death.
Two HPA laboratories and six other laboratories across the UK used the affected kits between November 2010 and September 2011. The affected patients are all from England and Scotland.