The Health Sciences Authority (HSA) has overseen the recall of 584 faulty HIV screening kits, each containing 25 tests.
The recall was carried out because some people in the early stages of an HIV infection could have received false negative results.
HSA said that of the recalled kits, potentially 547 had been used. As each kit can be used for 25 tests, up to 13,675 tests could have been carried out. However, HSA clarified that some patients would have returned for retests.
The faulty rapid screening kits, from Standard Diagnostics, test for HIV with blood drawn from a pricked finger. They then give results in around 20 minutes.
Get The Straits Times
newsletters in your inbox
The faulty kits are less sensitive. HSA said that this means when a patient is in the very early stages of an infection, the result might be a false negative - telling an infected patient that he or she is free from HIV.
However, if an infected patient had gone for a follow-up confirmation test at least three months after being exposed to the HIV virus - as is standard practice - he or she would have correctly received a positive result despite the faulty kits being less sensitive.
How the test works
The faulty products were from the SD Bioline HIV Ag/Ab Combo, a rapid finger-prick screening kit .
Generally, people who suspect they may be infected go for this type of test after a window period of four weeks to three months after they were potentially exposed to HIV.
A small amount of blood is taken from a finger by pricking it - hence the name. The blood is transferred from the finger onto an indicator tray.
It is then mixed with a special liquid. After waiting for a short while - about 20 minutes for this particular test kit - certain lines will appear on the indicator tray.
Depending on the specific test being used, the number of lines as well as their position will tell the healthcare professional whether or not the patient is HIV positive.
Local importer Unison Collaborative conducted the recall of the faulty test kits. It issued a notice on May 11 to 26 clinics and one hospital here which were supplied with the affected kits.
It advised them to stop using the kits and recommended retests for those who had tested negative. These tests are provided at some healthcare facilities for a fee.
HSA said that all high-risk patients who use any brand of rapid test kits are routinely advised to return for another test to confirm their negative status.
Action for Aids, one of the affected healthcare providers, said it received 6,825 tests and had already used all of them when it was notified.
However, its general manager, Ms Sumita Banerjee, said that if its clients had followed the standard procedure of doing a confirmation test three months after the time they were potentially exposed to HIV, the faulty product would not have affected any of them.
"We would advise clients who feel that they have been affected by the recall and have not done a confirmation test to come to the clinic where they can be tested again and follow the standard protocol."
She clarified that the test kit that UK Aids patron Prince Harry saw when he visited on Sunday was from a different manufacturer.
Correction note: An earlier version of this article said that affected patients will not need to pay to retake the test. This is incorrect. The Health Sciences Authority has clarified that the tests are provided at some healthcare facilities for a fee.