Last week the medicines regulator (MHRA) refused to approve mass daily testing in schools, yet next week teachers will be required to take virus tests twice a week, so what has prompted this apparent U turn? The same regulator passed emergency authorisation last December which gave permission for the public to use lateral flow tests without any prior medical training. That signalled the government wants us to use mass screening for the virus to help exit lockdown. In the absence of other self-use tests the lateral low test (LFD) is the only one available and the government has invested heavily in it.
Following a mass screening test in Liverpool last year the lateral flow test was found to miss up to 60% of positive virus cases, so its value was questioned and the roll out put on hold. It was nevertheless used by universities to release students for Christmas despite concerns that the test failed to detect cases with a low viral load and thus gave too many false negatives. Both the manufacturer and supporters claimed it was still a good way to detect virus carriers who would otherwise be free to spread it widely if not alerted to isolate.
The LFD is effective at catching people who are infectious, but it is an antigen test and only looks for protein antigens that live on the virus surface. These are present where there is a high viral load, but absent when the load is light at the start or end of infection. An antigen test will reveal whether your immune system has created antibodies, it thus tells if you have been exposed to the virus at some level. A negative test means you don’t appear to have the virus and are unlikely to be a transmitter, but a positive test means you will have to isolate and take a PCR test a more thorough test.
A polymerase chain reaction (PCR) test detects all forms of the virus; it picks up pre- and post-viral infectiousness, it detects light as well as heavy viral loads. It requires a laboratory analysis and from test to result takes time. As it identifies all forms of the virus it can sometimes ‘over shoot’ to deliver more false positives, in contrast to the LFD which tends towards false negatives, neither is perfect. Nevertheless PCR is the gold standard recognised by all international airports and airlines. The fastest turn around offered by a PCR lab is three hours, but this is only if you are flying out of Berlin.
During the first lockdown we were promised a simple antigen test using a finger prick blood sample, but this never materialised. The authorities felt it was unreliable and it was never mentioned again. Antigen testing, we were told then, was for identifying whether you had built up some immunity to the virus, talk of it dropped from the agenda once the first wave took hold. Now that so many people are asymptomatic and the virus is rampant an antigen test will identify where it has reached. A more comfortable and simple saliva test has been on trial for many months, it should be due for launch soon, but like the blood sample test it has gone very quiet, LFD is the only test in town.
On 22 January new research out of Oxford showed that the LFD picked up 83-90% of infections, although it didn’t qualify the load level to which this applied. The manufacturer’s initial claim was of 84-96% of infections with high viral load, but this was questioned after the Liverpool trial. The new Oxford paper encourages the government to press ahead with mass screening and self-testing. The LFD test is being used in hospitals, care homes, schools and local authorities, and UK based manufacture is soon to start. Sampling error due to incorrect self-sampling of nose or throat remains a problem, but at least a result is available within a couple of hours, irrespective of accuracy.
