LFT (Lateral Flow Test) is currently being used for asymptomatic screening (e.g. of health or care home staff), to test to see if someone who seems well might be carrying infection. It is less likely to give positive result for someone who is infected but not infectious, but it is not ideal for sick patients, as it doesn’t always detect the virus when too little is present to result in infectiousness. A big advantage is that results are available within 20-30 minutes. Therefore, LFTs can be used if it is necessary to check if someone might possibly be infectious, maybe before visiting an elderly relative.
PCR is best used to test if someone who is ill has contracted COVID. It is less likely to miss evidence of actual infection, but unfortunately it can often inaccurately label someone as being infectious. In particular, PCR can detect minute fragments of non-viable virus for several weeks after an infection, when the person is no longer able to pass it on, or detect virus where so little is present that a person is not infective. WHO now advise against using PCR for screening programmes. PCR tests should only be used to check if a sick person actually has COVID.
PCR tests are described as ‘99% accurate’. This sounds good, but remember if about 1% of people tested are genuinely infected and a further 1% give a false positive result, that is an apparently positive result when there is no infection present; then a positive result has only a 50:50 chance of being right. There is also a very serious practical problem with PCR tests. They have to be sent to a laboratory and often take a couple of days, maybe more, to come back. The delays this causes in starting to trace any contacts is far more damaging than any gain from PCR tests possibly detecting the very few infectious cases with too little virus present to be reliably detected by LFTs.
LFT and PCR tests are said to have similar rates of false positives. The exact figures are not clear, but the various mass testing programmes indicate that false positive are really not a significant issue for LFTs, whereas although PCR tests may in principle have similar false positives, in practice they can have a much higher rate than LFTs, due to imperfect laboratory processing. It is therefore important to realise that when used for screening people without symptoms many apparent positive results may be false.
Mass screening 1000 hospital staff with PCR every week results in maybe 10 positive results every week from staff who are either not infected or are infected but never become significantly infective (due to not enough virus being present), meaning 10 key staff have to self-isolate for 10 days, and every week will be a different group of non-infective positives, so you can see how mass testing can rapidly cause problems with staffing. Moreover, it is not just the 10 false positives who have to self-isolate but also their close contacts, which may result in whole departments being sent home. Mass screening with LFT instead of PCR would reduce but not eliminate this problem.
A better solution, as staff absences for contacts is greatly exacerbating pressures in the NHS, would be daily LFT tests for key workers who have been in contact with an infected person. That would enable them to continue working.
Recovery Medical & Science Group