SINGAPORE - Coronavirus tests are conducted by 13 laboratories in Singapore, the majority of which are located at public healthcare institutions such as hospitals, the Ministry of Health (MOH) has said.

It added on Wednesday (May 13) that test results are checked against patients' clinical symptoms, and that it regularly audits these results, including those from private laboratories.

Earlier this week, MOH had said there were 33 confirmed cases which turned out to be false positives, with patients retested and found to be negative. Following this, the laboratory stopped testing to recalibrate its test kits and revalidate them to make sure that results are accurate.

MOH's director of medical service, Associate Professor Kenneth Mak, said the error had occurred because a test kit used on an analytical machine at the laboratory which conducted the tests between May 5 and 9 had not been optimally calibrated.

This resulted in test results not being interpreted correctly.

"Currently, no other laboratory is using the same test kit and analytical instrument combination," MOH said in its statement on Wednesday. "The laboratory has switched to another test kit that has been proven to work on the analytical instrument."

It added that the affected individuals were isolated while assumed to be positive and, although they have since been tested negative, will be further tested as necessary.

The error meant that three of the four Covid-19 cases found among healthcare workers at the Singapore Expo community care facility had actually been cleared of the virus.

It also led to a factory-converted dormitory at 45 Kaki Bukit Place being wrongly identified as a cluster.

In the week prior to the incident, Prof Mak had said that around 7,500 tests were being conducted daily.

The ministry will issue advisories to labs to ensure that test results are accurate, although equivocal results may emerge from time to time, it said.

In such cases, laboratories will be required to carry out repeat tests with a comparable test kit if possible or request a new sample from the patient. If the issue still cannot be resolved, the samples are sent to the National Public Health Laboratory for confirmation.

The laboratory, which was set up in 2009, helps to track infectious disease outbreaks such as Zika or the H1N1 swine flu pandemic.

"As a precautionary measure, public health measures such as contact tracing may be instituted for cases with equivocal results while pending verification, and they are isolated," MOH said.