Can I get a false negative PCR test? How reliable Covid tests are after 43,000 people given incorrect results

Asia's Tech News Daily

More than 40,000 people infected with Covid-19 were wrongly told they did not have the virus due to an error at a testing laboratory, health officials have said.

NHS Test and Trace has suspended operations at a lab in Wolverhampton run by Immensa Health Clinic Ltd after an investigation revealed the apparent blunder.

The UK Health Security Agency (UKHSA) – which has replaced Public Health England – has been investigating reports of people testing negative on PCR tests after receiving positive lateral flow results.

It estimates that around 43,000 people were incorrectly recorded as testing negative between 8 September and 12 October. Most of these cases were in the South West of England, but some were recorded in the South East and Wales.

A number of possible causes had been speculated for the unusual results, from a new Covid variant to mucus from snotty noses affecting tests.

But it now appears the lab error is to blame, and that more than 40,000 people could have been spreading the virus to others after being told they did not have it.

How accurate are PCR tests?

PCR tests are the most reliable and accurate way to diagnose Covid-19.

The exact accuracy of the tests is not known, as there is no gold standard to compare them against, but trials have shown them to be up to 99 per cent accurate.

Will Welfare of UKHSA said: “We have recently seen a rising number of positive LFD results subsequently testing negative on PCR.

“As a result of our investigation, we are working with NHS Test and Trace and the company to determine the laboratory technical issues which have led to inaccurate PCR results being issued to people. We have immediately suspended testing at this laboratory while we continue the investigation.

“There is no evidence of any faults with LFD or PCR test kits themselves and the public should remain confident in using them and in other laboratory services currently provided.”

How accurate are lateral flows?

There has been a lot of debate around how accurate lateral flow tests are, and concerns around false positives.

There are two main ways of measuring the accuracy of tests – sensitivity and specificity – which are defined as follows:

Sensitivity: The ability of the test to accurately diagnose people who are infected with the virus.

Specificity: The ability of the test to accurately diagnose people who are not infected.

A recent Cochrane Review, which combined the results of multiple studies assessing the accuracy of lateral flow tests, found the specificity of lateral flow tests to be very high – around 99 per cent – meaning if you test positive it is very likely that result is true, and that false positives are very rare.

Their average sensitivity was 72 per cent among people with Covid symptoms, and 58 per cent for people without symptoms.

This means for every 100 people infected with Covid, and displaying symptoms, 72 would show up positive on a lateral flow test. For every 100 people infected but not displaying symptoms, 58 would show up positive.

However, a newer study published this week has found lateral flows are more than 80 per cent accurate at identifying positive cases, and that rises to over 90 per cent for people who are most infectious.

Professor Michael Mina, from Harvard School of Public Health, said lateral flows can “catch nearly everyone who is currently a serious risk to public health”.

What should I do if I test positive on a lateral flow?

Anyone who tests positive with a lateral flow – which are usually done at home and checked only by the person taking them – is advised to self-isolate and arrange for a PCR test, either posted to their home or taken at an external site.

The official guidance states that if the PCR is negative, they can end self-isolation and continue to act as if they are not infected.

However, a regional public health official has said people should continue to isolate even if they receive a negative PCR result if they are experiencing symptoms.

Becky Reynolds, director of public health for Bath and North East Somerset council, told the BBC: “The advice is also to think about your local situation, do an individual risk assessment – so what is the likelihood that even though the PCR is negative, that you may still have Covid?

“If thinking it through there is quite a chance you have Covid, even if the PCR is coming back negative, then regard it as Covid and self-isolate.”

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