More Than Half of Positive Test Results in Schools Likely to be False Positives

Photograph: Dave Hunt/AAP

Children returning to school on Monday face being subjected to extensive rules and guidance designed to make the school ‘Covid-secure’. This includes frequent and regular testing of all students and staff. One mother, healthcare professional and reader of Lockdown Sceptics wrote in to tell us why her children will not be taking part.

Mass testing using lateral flow tests will be rolled out across all secondary schools across England from Monday March 8th as all pupils are [deep breath] going back to school. A test is required before each child is allowed to return followed by two further tests between three and five days apart in school. So, three tests in the first week. This is then followed by biweekly testing at home from thereon. “Testing remains voluntary but highly advised” (Department of Health, NHS Test and Trace). I have two children who attend a large comprehensive secondary school. There are just under 1,500 students in the school – if each child takes the three tests in the first week that amounts to 4,500 tests (plus the teachers). If my research tells me right there are just under 3.5 million pupils in state funded secondary schools in England so that is 10.5 million swabs in week one alone, plus the teachers (and plus all the other swabs that are being done elsewhere). Each lateral flow test is a single use disposable plastic swab. I am not sure how the swabs are disposed of but I can be certain they are not recycled – I expect they are incinerated as clinical waste.

My husband and I are both ‘front line’ health care professionals and have opted our children out of testing in key worker school and have indeed opted out for the next phase of testing starting next week. Indeed we have also opted ourselves out of testing at work (also not mandatory). This is because we have many concerns about the flaws of mass testing asymptomatic people.

Bear with me here. The lateral flow tests have been reported to have a specificity of 99.6% (Preliminary report from the joint PHE Porton Down and University of Oxford SARS-CoV-2 LFD test development and validation cell, 2020). That is to say that if you use a lateral flow test on 1,000 people *known to definitely not have Covid* then it will test negative in 996 of them. Meanwhile the sensitivity of lateral flow is variable depending on the person administering it, ranging from 76% when trained healthcare staff are the operators but falling to 58% when used by self-trained members of the public. Put another way: if 100 people who are *known to definitely have Covid* are tested with lateral flow, the result will be positive in somewhere between 58 and 76 of them. However this is only half the story when it comes to screening a population that has no symptoms. To understand the value of lateral flow when it’s used in this way, we need to understand the idea of Positive Predictive Value (PPV) and Negative Predictive Value (NPV). These are the values that answer the questions “I’ve tested positive, so what are the chances I actually have Covid?” and “I’ve tested negative, so what are the chances I really don’t have Covid?”. The trouble is that the PPV and NPV vary depending on the prevalence of the condition you’re testing for – if the condition you’re hunting for (in this case Covid) by using a given screening test (in this case lateral flow) is rare, then the PPV of the test will drop while the NPV improves – in other words a positive test is less likely to mean you really do have the disease, while a negative test is more likely to mean you don’t have the disease. Conversely if the condition is common then a positive test is more likely to mean you really do have the disease (i.e., the PPV increases) whilst the NPV drops.

Take my children’s school as an example: approximately 1,450 pupils. Let’s say six of them have Covid (based on the current estimated national average of 1 infection per 230 people – although in fact the estimated prevalence in my local area is even lower). Let’s also say that 66% of those will test positive (as per the stated sensitivity of lateral flow). That’s four positive tests and two negative tests amongst the children with actual Covid. This leaves 1,444 without Covid. 99.6% of them (i.e., 1438 pupils) will correctly test negative, so six will test positive. Therefore there are ten positive tests amongst the whole school, of whom four actually have Covid and six actually don’t. Some rudimentary maths will show that this equates to a PPV of 40%. In other words any pupil receiving a positive test has a less than 50/50 chance of actually having Covid. The reassuring thing is that based on these numbers a negative test is 99.9% likely to mean a child truly doesn’t have Covid, but the price of this ‘reassurance’ is a few unlucky children being removed from school (again) and put through another unpleasant and invasive test, not to mention the possible risk of anxiety, the impact on their family having to isolate whilst the PCR test results are awaited, but also the environmental impact. This is one of my main concerns – the environmental impact of using millions of disposable plastic swabs each week ( has a daily/weekly count available – at the time of writing there were 4,513,953 tests carried out over the last 7 days. This is a combination of lateral flow and PCR tests). The number of tests is going to rise dramatically over the next week and this is of huge concern to us as a family. I have raised this as a concern in a recent medical webinar. I was not completely alone however I genuinely think not many people have thought about this issue (someone did raise concern related to the types of plastics used in the swabs). This is at complete odds with the environmental campaign against single use plastic and the effects on our planet. Even more ironically children are taught extensively at school about protection of the planet and individual responsibility. What are we teaching them now?… I will leave the PPE and wipes and disposable masks and plastic EVERYWHERE for now as I am starting to sweat … and the children want to watch David Attenborough A Life on Our Planet

Stop Press: Covid Testing in school has been recommended by the Government, but is not mandatory, as we have previously reported. Nevertheless, most schools are doing their best to test all their pupils and according to a survey conducted by ITV this is eating up huge amounts of time that will take away from teaching time in many schools. Only 9% of schools have consent for the tests from all parents, and a majority are struggling to get consent from more than half. Perhaps because it would have teachers lives easier, 80% of them believe that testing should have been made compulsory by the Government.

Stop Press 2: The other core component of the guidance is on mask-wearing in the classroom. The Telegraph reports on the UsForThem letter to Gavin Williamson asking him to publish the evidence showing that masks interrupt transmission or abandon the measure. Meanwhile, 32 Tory MPs have demanded that the measure be dropped after Easter.

Stop Press 3: A teacher has written in to describe an unanticipated difficulty with ‘remote learning’.

My head of department went AWOL the other day at one point because she fell victim to a sophisticated online scam while teaching online. The scammers purported to be from her internet service provider fixing an issue with her connection (which they simulated) and sneaked into her phone by the back door, emptying her bank account in the process. Had she not been trying to teach an online lesson at the time it wouldn’t have happened – she of course was desperate to get ‘the problem’ sorted as fast as possible. Luckily she got the money back from the bank but it’s a mark of how vulnerable relying on multiple internet access can be with all the distractions of trying to manage online teaching, to say nothing of the potential for far more sinister hacking impacting on the children involved. For this teacher going back to ‘real’ teaching can’t come soon enough.

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