Lab service dismisses fears about 'false negative' Covid-19 tests

28 June 2020 - 15:39 By Tanya Farber
The National Health Laboratory Service has dismissed claims that there is no clinical value in the health department's strategy to test Covid-19 swabs jammed in the backlog.
The National Health Laboratory Service has dismissed claims that there is no clinical value in the health department's strategy to test Covid-19 swabs jammed in the backlog.
Image: MICHELE SPATARI/AFP

The National Health Laboratory Service has dismissed claims that there is no clinical value in the health department's strategy to test Covid-19 swabs jammed in the backlog.

A group of scientists from Wits University and the University of Cape Town released a public statement last week in which they said the chances of false negatives were too high with older samples.

Professors Marc Mendelson from UCT and Shabir Madhi from Wits were among scientists who said tests relied on the virus’s genetic profile, which starts to degrade after a few days.

They said that this “heightened the chance of a false negative” significantly and that any test swab older than two days should be discarded.

But NHLS spokesperson Mzimasi Gcukumana told TimesLIVE: “The backlogs in most provinces have been eradicated and the only backlogs at present are in Gauteng, Eastern Cape and KwaZulu-Natal.”

In Gauteng, “efforts are under way to manage the backlog”, which stands at 30,000 tests, and the backlogs in the Eastern Cape and KwaZulu-Natal had been “significantly reduced”.

He added, “We are aware of the theory that samples beyond a certain age can provide false negatives but our testing results do not support this theory.

“An analysis of the previous backlog in the Western Cape has proved that at 14 days after sample collection, there is still a mean positivity rate of 22%.”

Gcukumana said an initial study by the NHLS showed that if samples are stored at 4ºC,  they can “reliably provide accurate positive results 14 days after the sample was collected”.

Furthermore, the NHLS’ “mandate is to provide health laboratory and related public health services to all public health care providers and other government institutions; and as such it has a legal obligation to provide such service if requested to do so”.


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