19,000 Schoolchildren Will Needlessly Self-Isolate Every Week Under Government Mass Testing Scheme

Morten Morland’s cartoon in yesterday’s Times

Yesterday Lockdown Sceptics reported that the Government is planning to require parents to test England’s three million secondary schoolchildren twice a week. I suggested that because lateral flow tests (LFTs) are being used for this intrusive testing regime, 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. A reader got in touch to say, while technically true, we should certainly not underestimate how many false positives will 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 the false positives. Since there are 3,448 secondary schools in England, that’s five or six children in every school in England every week. I’ll let him explain.

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) – probability that a positive is not a true positive.

If we assume that all three million schoolchildren are tested twice a week (so six million tests weekly), then 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% on February 18th) and the REACT study (0.51% for the period February 4th-13th) we get the following.

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

Prevalence (%) Number of True Positives (TP) Number of False Positives (FP) Number of False Negatives (FN) False Discovery Rate (FDR) Positive Predictive Value Negative Predictive Value
Zero Covid-19 0.000 0.00 19200.00 0.00 100.00% 0.00% 100.0000%
ZOE App 0.334 19038.00 19135.87 1002.00 50.13% 49.87% 99.9832%
REACT study 0.510 29070.00 19102.08 1530.00 39.65% 60.35% 99.9743%

So, between 40% and 50% 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%), which is likely to be more accurate/realistic when parents are doing the testing, we get:

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

Prevalence (%) Number of True Positives (TP) Number of False Positives (FP) Number of False Negatives (FN) False Discovery Rate (FDR) Positive Predictive Value Negative Predictive Value
Zero COVID-19 0.000 0.00 19200.00 0.00 100.00% 0.00% 100.0000%
ZOE App 0.334 11623.20 19135.87 8416.80 62.21% 37.79% 99.8590%
REACT study 0.510 17748.00 19102.08 12852.00 51.84% 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.

If only there was another way…



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Author: Weaver

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