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Why mass testing for pupils cannot work

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IT’S been reported  that the Government is planning to require parents to test England’s three million secondary schoolchildren for coronavirus twice a week. 

Apart from the highly questionable ethics involved, the use of unreliable testing will have highly damaging unintended consequences that government seems not to have thought through. I had thought that that because lateral flow tests (LFTs) are being used for this intrusive testing regime and not PCRs, the number of false positives (positive test results for people who do not have the disease or are not infectious) would be lower than with PCR tests.

However a reader has been in touch to say that whilst this is technically true, we should not underestimate how many false positives will still be produced by this frequent mass testing. He calculates that more than 19,000 schoolchildren every week, plus their families, classmates, teachers and other contacts, will be forced to self-isolate needlessly because of false positives. Since there are 3,448 secondary schools in England, that’s five or six children in every school in England every week. He explains here:

The reality is that LFTs produce very high numbers of false positives when used in the mass screening of asymptomatic populations. To be fair to the companies, these tests were developed, tested and licensed for use in symptomatic patients presenting at hospitals, where they have very high diagnostic value. They are not appropriate for use in asymptomatic patients where a false positive adversely affects numerous family members and other contacts as well.

Of course, PCR tests have their own problems, really are not a gold standard (the way they are used), and are badly abused. But that is a different story. 

The number of false positives depends on the prevalence in the community. Note: False Discovery Rate (FDR) means the probability that a positive is not a true positive.

If we assume that all three million schoolchildren are tested twice a week (six million tests weekly), even in a population with zero Covid-19 the Innova test will still find 19,200 positives weekly (all false positives, of course, because there is no COVID-19 in the population). If we use the prevalences specified by the ZOE app (0.334 per cent on February 18) and the REACT study (0.51 per cent for the period February 4 to13) we get the following.

Six million tests per week, sensitivity 95 per cent, specificity 99.68 per cent

 Prevalence (%)Number of True Positives (TP)Number of False Positives (FP)Number of False Negatives (FN)False Discovery Rate (FDR)Positive Predictive ValueNegative Predictive Value
Zero Covid-190.0000.0019200.000.00100.00%0.00%100.0000%
ZOE App0.33419038.0019135.871002.0050.13%49.87%99.9832%
REACT study0.51029070.0019102.081530.0039.65%60.35%99.9743%

So, between 40 and 50 per cent testing positive will be false positives (depending on which prevalence you take) – almost half of positives are false positives.

If we use the sensitivity identified in this BMJ article for self-trained members of the public (58 per cent), which is likely to be more accurate/realistic when parents are doing the testing, we get:

Six million tests per week, sensitivity 58 per cent, specificity 99.68 per cent

 Prevalence (%)Number of True Positives (TP)Number of False Positives (FP)Number of False Negatives (FN)False Discovery Rate (FDR)Positive Predictive ValueNegative Predictive Value
Zero Covid-190.0000.0019200.000.00100.00%0.00%100.0000%
ZOE App0.33411623.2019135.878416.8062.21%37.79%99.8590%
REACT study0.51017748.0019102.0812852.0051.84%48.16%99.7845%

So, between 50 and 60% testing positive will be false positives (depending on which prevalence you take) – a majority of positives are false positives.

In summary:

False positives in mass screening are not rare – they are very common (relative to the number of true positives). Too much emphasis has been placed on the false negatives in the MSM but, for a disease that is as bad as a bad flu, the false negative rate can be ignored when the prevalence is quite low. This would not be true of Ebola or smallpox of course, but Covid-19 can hardly be compared with these.

Substantial numbers of false positives will be generated as large scale testing of schoolchildren is rolled out. The proportion of false positives to true positives will greatly increase as the community prevalence decreases. 

It should be clear that the country will never be able to meet the goal of fewer than 1,000 ‘new cases’ per day in order to remove restrictions.

This article appeared in Lockdown Sceptics on February 19, 2021, and is republished by kind permission.

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Will Jones
Dr Will Jones is a maths graduate with a PhD in political philosophy and author of Evangelical Social Theology: Past and Present (Grove, 2017).He blogs at https://faith-and-politics.com/

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