The dental clinic at the University of Hong Kong is calling on 250 patients to take blood tests for HIV and hepatitis B and C after discovering equipment had not been properly sterilised. The patients, including students, staff and their families, all underwent procedures last week at the clinic. More than 100 patients have been contacted since a member of staff discovered on Friday that a key step in disinfecting instruments had been skipped for four days in a row. Dentists and doctors say the chances of HIV infection is low, but hepatitis B poses a higher risk as the virus can survive on dental tools for up to a week. Michelle Wong, an arts student who had her teeth cleaned on Friday, said she was worried. "This is unbelievable. I only had dental cleaning, not even something complicated like root canal treatment or removing wisdom teeth, but now they say I need a blood test," she said. "I will not be using the dental service there for a while." The clinic apologised for the blunder yesterday. "[The HKU health service] has reported the incident to the Department of Health and wishes to extend its sincere apologies to patients concerned," it said. The health service issued a statement last night in response to an inquiry from the Sunday Morning Post , which had received a complaint from a patient about the incident. The 250 patients were treated between Tuesday and Friday at the clinic on the Pok Fu Lam campus, it said. "Some dental instruments were found not to have gone through all the procedures of sterilisation," it added. The blunder was noticed by a nurse, who found the labels of some instruments were not of the right colour, suggesting they had not been put in an autoclave - a sterilising oven. Using the autoclave was an essential last step in cleaning dental tools because it was the only way to kill some strong viruses including HIV and hepatitis B, Dental Association president Sigmund Leung Sai-man said. "I have never heard of any dentist forgetting to use the autoclave in the 20-odd years of my practice," Leung said. "This is a rare and weird incident." HKU said it had consulted three medical experts, who considered that since the instruments had undergone the first three steps of a four-step disinfection procedure, the chance of viruses surviving was "very low". The university has set up a team to review the sterilisation procedures. It did not identify the staff responsible. Initial blood test results will be out in two days. Both Leung and medical council member Dr Choi Kin noted that HIV would be killed once exposed to air. But the hepatitis B virus could survive in blood stains for up to a week if the room temperature was high, Choi said.