For most of us they are no longer free, though at £2 a test they aren’t expensive. But the question is, are they worthwhile? To understand the answer to that you have to understand that lateral flow tests aren’t very sensitive. You have to have quite a lot of virus in your throat and nose to get a positive. This is quite different from the PCR test, which, in principle, can detect any amount of virus, however small.
In practice that means that your test will probably be negative in the first day or two after you catch the virus, but it gets more likely to be positive in the day or two before symptoms begin, and then during the first 5 days of the illness.
Let’s look at how this affects our decision in different situations:
- You have symptoms of a cold, or worse. Yes, definitely worth the £2 for a test. It will pick up 80% of cases. In other words, if negative, Covid is fairly unlikely (but it doesn’t rule it out). If you repeat the test on day 2 and 3 and it’s still negative, then it’s even more unlikely that you have Covid. And if the test is positive you don’t need a PCR – you have Covid.
- You feel fine but you are going to visit someone who is vulnerable. The most vulnerable person would be someone aged over 75, unvaccinated or vaccinated but without a booster in the last six months, and with some other condition that impairs their ability to fight infection. This is much more tricky. If you have the infection but have no symptoms, the lateral flow test has only a 20% chance of being positive. So in 4 out of 5 cases when you are infectious you will be falsely reassured by the test. I’d say it’s still worth doing but, if it’s negative, you should still do everything possible to keep the other person safe: meet out of doors, stay apart and especially don’t kiss, wear a mask and sanitise your hands before you go in.
- You are a contact. That means you have been close to someone who has subsequently tested positive, or developed symptoms, in the subsequent 2 days. Yes, do a test but not immediately. It’s pretty sure to be negative in the first two days. If you only want to test once, do it on day 5. But you will only be in the clear after ten days from the contact. In addition to testing you have a choice of three policies: isolate for 10 days from contact; or mask, distance and sanitise for 10 days from contact; or continue with life as normal, if you have no alternative. But in this case my view is that, if you chose the second or third options, it would be wrong not to warn everyone, before you come into close contact with them, that you are a contact.
- You’ve tested positive. Obviously you have the same three options as if you are a contact but the importance of isolating is far greater. And some people, e.g. GPs and their staff, are required to have two negative tests, 1 day apart, at least 5 days after their positive test. That’s not a bad rule for all of us to follow.
Those who are still paying attention will have a question. “If I have the virus, but there’s too little of it in my throat and nose to give a positive test, is it also too little to infect someone else?” Sadly, no. The study I’ve relied on for my stats shows that you can still pass it on, even when you are testing negative. That study can be found by searching on ‘BMJ Jonathan Deeks 2022’.
Final thought: because the lateral flow test depends on your getting enough virus on to your swab, don’t skimp the swabbing, however uncomfortable you find it. 15 seconds rubbing on one tonsil then 15 seconds rotating it deep in a nostril will give you the best result.