The National Dental Centre Singapore (NDCS) has contacted 697 out of the 714 patients who may have been treated with equipment that had not been fully sterilised.

Of this number, 115 visited the centre for further clarification after receiving the call from NDCS telling them of the incident.

It is also doing free blood tests for 92 of the patients who had requested them, to check for human immunodeficiency virus (HIV) and hepatitis B and C. However, this test is merely to find out if they had already been infected prior to their dental treatment earlier this month. They will be given a second and third blood test in September and December respectively to check if they might have been infected at the dental clinic.

So far, the results are in for 88 patients. None has HIV, but 18 had previous hepatitis infections with two who are carriers.

Hepatitis B is quite common here, with one in five people having been infected. The most common transmission is from mother to child at birth, or through unprotected sex. Singapore introduced national childhood immunisation against hepatitis B in 1987.

On June 5, the NDCS discovered that one batch of equipment had gone through only two of the three sterilisation steps. It recalled all the equipment, but 72 packs had been used, both on the day of discovery and the following day.

It issued a statement last week to say it would be contacting all the patients treated in clinics where these packs were used, though it stressed that the risk of infection is extremely low.

That is because the packs were part of a batch that had gone through two of the three steps normally used to sterilise equipment that is to be reused, but not the final steam sterilisation to destroy bacterial spores.

The NDCS said the first two steps, thorough machine washing and thermal disinfection, would have "removed or inactivated almost 100 per cent of all organisms".

These include hepatitis B and C and HIV, and bacteria such as Methicillin-resistant Staphylococcus Aureus. Since the risk of infection is "extremely low", the NDCS said screening was not necessary.

Of the two bacteria that could have been transmitted, there is no screening for one, and the other is mild and would resolve within a week. But it decided to provide the screening as some patients requested them.

The NDCS again apologised for the incident and the anxiety it has caused.