Nurse comment on false negatives

[thesplashing]

Last week my toddler had a horrible cough and I had a light tickly cough.

Got PCR's and toddler and DH tested negative.
Both my parents tested negative (were with them for an extended period of time). I tested positive and we are all isolating.

In the meantime toddler's cough isn't going away and temp has spiked a couple of times so I have contacted the GP for an appointment. On the phone I obviously mentioned we were still isolating due to my positive and the nurse said it was most likely a false negative pcr for my DH and toddler if I had a positive.

It's baffled me a bit as everyone I was in very close contact with is a negative, they can't all be false surely? I've seen threads on here where it was quite common for only some of the household to receive a positive. Are the PCR's that unreliable?

Not an expert but I thought the pcrs were fairly accurate and that it was the lfts that had false postives/negatives. Neither are going to be 100% though. I think the surgery are correct to be cautious though as it's a false negative is a possibility even if unlikely.

False negatives are definitely an issue, but it's also perfectly possible for only one person in a household to test positive.

I do think that its hard to do a good covid test on a toddler and if I was positive for Covid and my child had clear covid symptoms I would be inclined to assume that they did in fact also have covid.

The adults it's less clear. For all you know your husband could have had it asymptomatically and passed it to you and your toddler and so he is already testing negative again as he has recovered from it for example. Testing just isn't perfect when you're dealing with a virus that can be completely asymptomatic or have atypical symptoms.

PCRs are generally much more accurate than LFTs, though some false negatives are possible. The chances of them not being false negatives would be low though I'd have thought.

Dh and Dd didn't test positive or get symptoms when I had covid last autumn.
Dd has since had an antibody test and she came back negative, which backs her negative test up.

False negative rates for PCRs are difficult to quantify as we don't have better to compare them with but they could be as high as 50%.

It's more the swabbing that can be inaccurate, particularly with young children. I'd be tempted to assume your toddler probably is positive, given your positive, his symptoms and the fact that you have very close contact with each other.

As an example, a friend's child had symptoms and it was at a time when our area was encouraging whole households to test. The child with symptoms came back negative, but my friend and one other child came back positive.

@QueenStromba

False negative rates for PCRs are difficult to quantify as we don't have better to compare them with but they could be as high as 50%.

Nonsense. It's around 3% according to the ONS

There are quite a lot of false negatives with PCRs (I have heard 20% from a university and 50% from 111) but, as a PP said, they are difficult to quantify (what would you measure them against) and the government don’t release figures anyhow,

False positives, on the other hand, are very low with PCR tests.

And yes, it’s my understanding that the swabbing is the main problem - it might not pick up the virus. Home swabbing May be particularly bad as people are not good at ranking it right to the back of their noses etc.

Ramming

ONS say about 5% but that in that study subjects self swabbed supervised by a HCP which has been shown to be as accurate as if a HCP does it. In the real world the sensitivity is much lower.

This study is just one of dozens to find that real world sensitivity is nowhere near 95%.

journals.plos.org/plosone/article?id=10.1371/journal.pone.0251661

False negatives are possible with PCRs largely due to not very good swabbing, or swabbing on a day when you just didn't have as much virus around.

A colleague had Covid - all the symptoms, Covid appearance on X-ray, had antibodies afterwards. Definitely Covid. Never had a positive test but tested fairly late in the illness - it was before tests were readily available. He was told that if you don't test at the right moment you may get at negative test but doesn't mean it wasn't Covid.

Very interesting - thanks @Queenstromba.

My hospital has had inpatients with full-blown clinical Covid (ground glass opacacities in their lungs etc.) who never tested positive on a PCR.

It’s also had loads who had positive PCRs but later antibody tests were negative.

Opacities

Agree with @annamagnani it can be down to timing. In my house we all had covid around the same time, but 2/4 PCRs were negative first time round, even though we all had symptoms. They were positive when we retested 3 days later.

Doesn’t even necessarily have to be false negatives. Ineffective swabbing could be an issue, or if you all tested around the same time it could be others of you would’ve tested positive a couple of days later.

@QueenStromba

False negative rates for PCRs are difficult to quantify as we don't have better to compare them with but they could be as high as 50%.

@QueenStromba

I don't think an article from the first months of the pandemic tells us much about the current reliability of PCR testing. Protocols have very likely evolved from those used in the early days

Really interesting responses so far thanks.

DH wasn't allowed to get out of the car to administer the test to my toddler so they made him go into the walk in bit instead where he was watched doing it by one of the marshals so is fairly sure he did it correctly.

Also in the 4 days before symptoms and after all of mine and my husbands lateral flows were negative. I don't know if that shows anything? We were testing everyday as we were attending a wedding.

A friend became ill last Wednesday- LFT and PCR negative
Got worse and worse- multiple negative LFT
Finally PCR positive on Sunday.

Yep, above mentioned colleague went to A+E but wasn't admitted.

A+E doctor told him 'You know it's Covid, I know it's Covid, it is Covid. We see this all the time' - this was back in the first wave

Makes you wonder how many people with symptoms are cheerfully spreading Covid around having been falsely reassured by a negative PCR just because they did it a day to early or a day too late.

My next door neighbour had covid. He’s on day 9 now & the rest haven’t caught it so far. They’ve done 3 lots of PCR tests each (mum & two kids) 🙈 & none have symptoms either. They’ve not been isolating from him either as the kids are 4 & 2 and it’s not exactly easy to do so..

It does happen. 🤷🏼‍♀️

Yes - with the original variant only (from memory) around 30-40% of household contacts caught it. Even though delta is twice as infectious there will still be a minority of household members whp don’t contract it.

I would imagine the PCR results are pretty reliable if they are done properly. That's a big if. User error is an issue. With the uncomfortableness of the test and the human instinct to not shove stuff down your throat so far that you gag that user error must come into it I imagine.

Same thing happened to a friend of mine, they were positive and their partner became ill 3 days later, exact same symptoms, and tested negative with a postal PCR test when it was clearly covid.

AT Christmas my niece and nephew who live in outer London all were offered PCR tests at the end of term due to them being in tier 4. Niece aged 11 tested positive but no symptoms, by Christmas Eve her dad unwell and so the remaining family did PCR tests (my sis had slight cough) and these second PCRs all came back positive. Not sure if that helps or not. Just another example of 1)how covid infection variable how covid impacts on one household and 2) how can't rely on PCRs. I'd do a repeat PCR and self isolate.

My ex had a false negative in late January. Didn't recover, tested again in early February and was positive.

We (about 4 of us) all had covid symptoms in early January but all tested negative so carried on with life as normal. Looking back, we probably passed it on!

"I don't think an article from the first months of the pandemic tells us much about the current reliability of PCR testing. Protocols have very likely evolved from those used in the early days."

PCR is a mature technology so we had robust protocols early on. If anything there were likely fewer false negatives then as there was very little sequence evolution in the first six months or so of the pandemic. We actually estimate the prevalence of various variants by tracking the number of samples which test positive using one PCR protocol but negative on others.