PCR v LFT?

A question about PCR v LFT tests and maybe this is something I should already know.

My husband (Chronic lymphocytic leukaemia (CLL) patient) is having a small procedure next week at a private hospital – a couple of back injections for sciatic problems; it’s a day thing and it’s done in theatre, so all the covid protocols have to be followed. So, as part of the pre-assessment bundle they sent him two LFTs, with instructions to use one a couple of days before and the second on the morning of the procedure. Then he had a pre-assessment phone call – and when she realised he had Chronic lymphocytic leukaemia (CLL) she said to ignore the LFTS because he needs to have a PCR. So he now has to go for PCR 3 days before – and then isolate until the day (I also have to isolate, which is a bit of a nuisance – but what’s 3 more days after almost 2yrs of isolation😊).

So why the difference for blood cancer patients? I suspect it’s something to do with covid maybe incubating for longer in someone who is immunocompromised – and a friend (retired medical person) said she thought there are more cases of false negative LFTs in anyone immunocompromised? If that’s correct I hadn’t realised it; I thought the level of accuracy of LFTs was the same for everyone. Just trying to understand – and to know how much I can trust LFTs. Thanks.

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Hi@Ruth,just to say ,I dont have your husbands diagnosis i’m on watch and wait for MGUS .It was/is standard procedure for any invasive Hospital procedure ,any patient.Last November I had a procedure and same as your husband LFT then PCR and isolate prior for 3 days ,hope this helps. (I also had to be careful 2 weeks before hand !
All the best
Bannanacake

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Yes, it was standard procedure then - but not now, at least not at the private hospital he’s going to; until they realised he had Chronic lymphocytic leukaemia (CLL) it was two LFTs and no isolation!

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Hi @Ruth I think perhaps it depends on each individual organisation and it might also be a postcode lottery.
My husband (who does not have blood cancer) had a procedure a couple of months ago and others before that over the weeks before and he had to have hospital (NHS) drive through PCR tests and we both had to isolate for 3 days.
Perhaps that is a really good question to ask the hospital and please let us know what they say.
Wish your husband well for me and both of you look after and be kind to yourselves.

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I’ll try and find out when we go there for his checks. They were quite happy with the LFTs until Chronic lymphocytic leukaemia (CLL) was mentioned - then she went to check with her boss and when she came back she confirmed that ‘because of the leukaemia’ he had to go down the PCR route. I’ll let you know what they say - although I suspect she may not know the reason either!

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Firsty, it’s important to recognise that LFTs and PCRs are actually testing for completely different things. The LFT is reacting to surface proteins on the virus, and the PCR is actually hunting for the virus’s known genetic fingerprint, and because of that they have quite different diagnostic sensitivites and specificities.

It’s not that the accuracy of the LFT is different for different people, I think ultimately the reasons for this different policy are just that the PCR is much more accurate. The false negative rate (i.e. actual positive cases that are missed) for LFTs can be as bad as 50%, and some research suggests as bad as 70% for Omicron. PCRs will miss only about 5-10%, and can detect much lower amounts of the virus, much earlier on in the infection period. So, the PCRs are just more definitive, and are therefore the obvious choice for testing in patients where it’s really important to know (like blood cancer patients) as opposed to general asymptomatic surveillance where misses are more tolerable.

As an aside, it’s worth noting that the false positive rate (saying you have it when you actually don’t) on the LFT tests are very low, and it was this realisation that led to the need for a confirmatory PCR test after a positive LFT to be dropped. When either type of test comes up positive, you should believe it, but for the reasons above, that’s not the case if it comes up negative. If an LFT comes up negative, it does not mean you don’t have covid. It would be more correct to say that it “doesn’t show a positive” rather than saying it’s negative, whereas a negative on a PCR probably does mean you don’t have it.

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