Staff at a university dental clinic who failed to properly sanitise tools were guilty of "serious negligence", an internal investigation has found.
The lapses at the University of Hong Kong clinic exposed 254 patients to the risk of contracting hepatitis and HIV over four days. One patient has still not been traced, five weeks later.
However, the inquiry was unable to assign blame to any individual member or members of staff. Its report did not say if any staff would be penalised.
Staff at the clinic, on the university's Pok Fu Lam campus, missed at least four standard steps to clean dental tools after use, the investigative panel found.
They failed to use sterilisation equipment properly on a set of tools, and missed running two checks to confirm they had completed the process before the tools were reused, it said. Each set of tools, once sterilised in an autoclave, or sterilising oven, can be used safely for several days.
The panel was unable to establish which of the staff was responsible for turning on the autoclave on the afternoon of October 30, panel member Dr Yuen Kwok-yung said.
He said: "It is very hard to pinpoint who was responsible … as about eight staff were on duty that day. The employees said they could not recall who had performed the duty that afternoon, and I believe they were speaking the truth."
The chairman of the HKU Students' Union, Dan Chan Koon-hong, criticised the panel's failure to establish which staff member or members were liable for the mistakes, but welcomed the fact the investigation had been completed speedily.
The Department of Health said it would conduct regular audits of procedures at the dental clinic, including the requirement that staff ensure proper sterilisation had been carried out and the results documented.
The incident came to light after a nurse noticed surgical instruments had been improperly sterilised. The labels on some instruments indicated they had not been put in an autoclave.
More than 400 items were estimated to have been affected. The tools had been used to treat 254 patients between the afternoon of October 30 and the morning of November 2.
The university asked the patients it was able to trace to undergo blood tests for HIV and hepatitis B and C.
None has tested positive for any of the diseases.
The report said: "The incident was found to have been caused by a very rare lapse in monitoring by dental surgery assistants." It recommended dedicated staff be assigned to sterilise dental tools.