SINGAPORE — The laboratory which had initially confirmed 33 people to be infected with Covid-19 before discovering that they were “false positives” has switched to another test kit, said the Ministry of Health (MOH) on Wednesday (May 13).

The affected individuals had also been isolated while they were assumed to be positive and will be further tested as necessary, the ministry added in response to queries from TODAY.

MOH did not specify where they had been isolated nor identify the laboratory.

MOH first announced the discovery of the 33 false positive cases on Sunday. The next day, the false positives, as well as another two equivocal cases that were also later found to be negative, were removed from the total tally of cases. 

The false positives were due to a test kit that was “not optimally calibrated” in a laboratory, resulting in tests being interpreted incorrectly, said MOH director of medical services Kenneth Mak at a press conference on Monday.

This caused the test results to be interpreted incorrectly. Some of the required testing steps were also “not completely followed through”, Associate Professor Mak had added.

The test kit in question had been deployed between May 5 and May 9, MOH said on Wednesday.

The ministry has since reviewed the test results of the affected batches, and there were no “false negative” results discovered from the review.

It added that no other laboratory is currently using the same test kit and analytical instrument combination.

“The laboratory has switched to another test kit that has been proven to work on the analytical instrument,” it added.

TEST RESULTS REGULARLY AUDITED

The health ministry said there are 13 testing laboratories conducting Covid-19 tests in Singapore, with “the large majority” done by the National Public Health Laboratory and the public acute hospitals, supported by various private laboratories.

“MOH regularly audits the test results of these laboratories. Laboratory test results are also checked against the patients’ clinical signs and symptoms,” said the ministry.

It added that equivocal results, which are “weak positives” or test findings that are borderline cases, occur from “time to time”.

In such instances, laboratories have a workflow for verifying these results, including repeat testing with another comparable test kit if it is available, and requesting for a new sample to be collected.

“If the issue cannot be resolved at their end, the samples are sent to the National Public Health Laboratory for confirmation,” said MOH.

“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.”