Does “testing” tell you if you are infectious?

Statement

Concerns have been raised about the Government’s mass testing strategy (see: Are PCR tests fit for purpose?). Even after the huge investments of the past year and high hopes in the field, there are still fundamental questions about the basic efficacy of some of the take-at-home testsPCR testing and the accuracy of lateral flow device (LFD) tests, also known as antigen tests.

In March 2021, a systematic review was published in the Cochraine Library’s Database of Systematic Reviews (systematic reviews are used, particularly in health research, to identify, evaluate and synthesise all the empirical evidence that meets pre-specified inclusion criteria to answer a specific research question).

The review reported that “in people with confirmed Covid‐19, antigen tests correctly identified Covid‐19 infection in an average of 72% of people with symptoms, compared to 58% of people without symptoms. Tests were most accurate when used in the first week after symptoms first developed ….”

A government evaluation, in association with Oxford University, found that the ability of the LFD test to correctly identify those without Covid-19 (called ‘true negatives’).  It found the lateral flow tests have a specificity of at least 99.9% (fewer than one false positive in every 1,000 lateral flow tests).

The analysis of the data, however, has been critiqued by some biostatistics experts who said that “when disease prevalence drops, even the most specific tests face problems as the number of false positives can outnumber the number of true positives – a scenario where a confirmatory test is absolutely essential to prevent large numbers being made to unnecessarily isolate.” The Government has since reversed the policy.

“We’re in this ridiculous state where people are being invited to test themselves twice a week with a test that’s really unreliable,” says Will Irving, professor of virology at Nottingham university, referring to the lateral flow devices. “If you get a negative result it doesn’t mean you haven’t got it, if you get a positive result, it doesn’t mean you’ve got it.”

Key Points & Evidence

This article from the Financial Times questions the efficacy of testing as scientists continue to battle over the accuracy of the take-at-home tests. In 2020, the debate centred on whether these devices missed a large proportion of cases, with some studies indicating they picked up only between 40 and 60 per cent of active infections. Proponents counter that they are good at picking up when individuals are actually “infectious”, rather than the periods pre and post infectiousness that PCR tests can also identify.

In recent months, as the prevalence of Covid-19 has decreased, attention has turned to whether these tests also tell people they have the disease when they do not — known as “false positives”. Screening experts have expressed concern that the vast majority of positive results given by the tests in areas where the prevalence of the virus is low are likely to be inaccurate. “In no other situation could you introduce a health tech or medicine without doing a health economic analysis,” says Irving at Nottingham university. “It might be costing £1m per case prevented — not only is that never calculated, but neither is any other alternative.”

Read more...on.ft.com

"Mass asymptomatic Covid-19 testing: Strategy and accuracy", House of Commons Research Briefing, 12 May, 2021

Concerns have already been raised in Parliament concerning the accuracy of the lateral flow devices (LFDs) and several government scientific committees have published several briefings on mass testing, covering their accuracy, use, the Government’s evaluation of LFDs, and an overview of scientific opinion on the value of mass testing.

Read more...commonslibrary.parliament.uk

"Covid-19: Lateral flow tests are better at identifying people with symptoms, finds Cochrane review", BMJ, 25 Mar 2021

Rapid antigen (lateral flow) tests are better at identifying Covid-19 infection in people *with symptoms* than in those with none, although the diagnostic accuracy of different brands of tests varies widely, a Cochrane review has found.

Read more...bmj.com

"Rapid, point‐of‐care antigen and molecular‐based tests for diagnosis of SARS‐CoV‐2 infection", Cochraine Review of Systematic Reviews, 24 Mar 2021

A study in March 2020 showed that people with confirmed Covid‐19, antigen tests correctly identified Covid‐19 infection in an average of 72% of people with symptoms, compared to 58% of people without symptoms.

So the lateral flow is effective in those who already have symptoms. The authors of the study also stressed that the number of samples from asymptomatic people “was around 10 times lower than from symptomatic people in the studies analysed, limiting the conclusions that could be drawn.

Read more...cochranelibrary.com

"Expert reaction to new analysis on Lateral Flow Device specificity", Science Media Centre, 10 March 2021

Read more...sciencemediacentre.org

Conclusion

If rapid antigen tests correctly identified Covid‐19 infection in an average of 72% of people with symptoms, compared to 58% of people without symptoms, does this mean that testing of non-symptomatic people is in any way effective or cost-effective?  Studies show that the prevalence of the virus (that is, having symptoms) would affect the accuracy of the results. What then is the point of testing when a person is anyway displaying symptoms (traditionally the indicator of a person being ill)?

 

At 0.5% prevalence, applying the same [antigen] tests in asymptomatic people would result in PPVs [positive predictive value i.e. the probability that subjects with a positive test result truly have the disease] of 11% to 28% meaning that between 7 in 10 and 9 in 10 positive results will be false positives, and between 1 in 2 and 1 in 3 cases will be missed.

Posted: 17th August 2021

Tagged: Testing

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