Are parents of positive lft school kids not bothering with pcrs?

[Bennetgirl]

I’m guessing most aren’t going by my two dds friends who have tested positive in school this week.

Do lft results count in the official figures if no pcr is done?

I know a number of parents who have taken a positive LFT as a positive and not gone for further testing simply because of how unpleasant it is for the children.

From parents I have spoken to, they say if a LFT is showing positive then the PCR will only say the same so see little point in putting their child through it again.

The school will be recording the results and sharing with the necessary people, but yes if DS had a positive I wouldn’t bother getting a pcr

I think LFTs only count if people bother to register them.

I suspect this is the main reason why they prefer PCR tests.

A friend dd tested +ve on a lft 3 days after her DM had a confirmed +ve PCR, they didn't bother to do a PCR and just continued to isolate for a few days longer. One reason was to avoid any further hassling calls from T&T and another was because she hadn't been in close contact with anyone outside the home that would have needed to isolate pre 16 August

We had at least three cases over the last 2 weeks of positive lft at school, and both were subsequently pcr tested and were negative. I was astonished as I heard this was v unlikely but one was our child and others are children of people we know v well, so I’m sure it’s true. School thinks there was some contamination or something.

My DC got positive LFTs, we did a PCR with the elder (came back positive). Not going to bother with the younger - they're both isolating from their positive test. It wouldn't change how we behave and we've made the school aware so I'm not sure what the point would be.

Talipes

Ons data from March shows that 60% of the positive lft tests taken when kids went back were false positives

So hundreds of thousands of kids/contacts isolated for sod all

This was proven by pcr

60% false positive!

We had several +LFT cases at my school, which then turned out to be negative PCRs. According to the testing team drinking any type of fizzy drink 30 mins prior to testing can result in a +LFT.

We didn’t, but it didn’t bode well with the school and they gave us a hard time :/

@Watapalava

Talipes

Ons data from March shows that 60% of the positive lft tests taken when kids went back were false positives

So hundreds of thousands of kids/contacts isolated for sod all

This was proven by pcr

60% false positive!

Whoah!!!

According to the testing team drinking any type of fizzy drink 30 mins prior to testing can result in a +LFT.
I thought you shouldn’t eat or drink anything at least one hour prior to LFT.

@Watapalava

Talipes

Ons data from March shows that 60% of the positive lft tests taken when kids went back were false positives

So hundreds of thousands of kids/contacts isolated for sod all

This was proven by pcr

60% false positive!

Can you link to this report please?

Thanks all. That’s interesting. The parents who don’t bother following up with a pcr could in fact have a negative and not know!

The opposite has happened at my kids school. Positive lft, took a pcr which was negative BUT kids had symptoms so isolated anyhow and took a further pcr which was positive. Medic parents on the whatsapp are commenting that it is common.

The rate of false positives on an LFT is only really an issue when infection rates are low (like in March). They are now much, much higher in kids, so false positives would be nowhere near the rate in March and are now more likely to be a true positive.

@Watapalava

Talipes

Ons data from March shows that 60% of the positive lft tests taken when kids went back were false positives

So hundreds of thousands of kids/contacts isolated for sod all

This was proven by pcr

60% false positive!

The statement here that there were 60% false positives on lfts at a particular point when cases were low has no relevance at the moment, when cases are high.

I'll illustrate with an example. The figures are not based on case data on any [articular date, but are for illustrative purposes.

For this illustration, I am going to assume that false positivity rates are 3 in 1000, and false negatives around 50% - approximately in line with initial Oxford University research in November 2020 when the lfts were self-administered by members of the public given a protocol, but simplified to make my Maths easier!

Let us take 3 scenarios:

1. True case rate is 4 in 1000, and everyone takes an lft.

The false positivity rate means that 3 of the 1000 will test positive at random. The false negative rate means that 2 of the 4 are not detected.

5 positives are found. 3 are false positives, 2 are true positives. 2 cases are missed.

1. True case rate is 40 in 1000

False positives are still 3 in 1000. False negatives mean that 20 of the 40 are not detected.

23 positives are found. 3 are false positives. 20 cases are missed.

1. True case rate is 400 in 1000 (OK, so this is unrealistically high, but it makes the point for the illustration)

False positives are still 3 in 1000. false negatives mean that 200 of the 400 are not detected.

203 positives are found. 3 are false positives. 200 cases are missed.

So when cases are low , false positives are an issue. When cases are high, as they are at the moment, false positives are much less of an issue than false negatives in terms of distortion of the data.

Either watapalava has not understood this, or they are deliberately selecting a point at which case rates were low to make a false general claim about the impact of false positives on lfts.

Apologies, I over-simplified and have come back to be a bit more precise!

When cases are 4 / 1000, the number of false positives is very near to 3 (but not quite, because it is 3 / 1000 and we have only 996 true negatives)

When cases are 40 /1000, the number of true negatives is 960, so again saying that the number of false positives is 3 over-states it slightly.

When cases are 400 / 1000, the number of true negatives is now only 600, and so, as a poster on another thread rightly said, the number of false positives is lower, at just under 2.

It doesn't affect the overall conclusion, which is that at high case numbers, false positives are a small (and slightly reducing) issue in terms of understanding the data, whereas when case numbers are very low, they are a much more significant complicating factor.

Two things.

1. false positive lft (not uncommon)

2. if you've "definitely" had covid, then the rules regarding testing change. At my school we are told not to bother with testing again within 90 days of a positive pcr. So every time the child developed covid like symptoms within the next 3 months (ie cough/cold symptoms!) then if they had already had pcr confirmed covid, then they wouldn't bother testing again, they'd just assume standard cough/cold and come into school as normal. But if they were only ever suggested positive on a lft and never actually confirmed, then they'd have to go for a pcr.

So basically, if you're fairly sure your child DOES have covid, then actually it probably works out better in the medium term to get them properly tested and confirmed, as it could well avoid further unnecessary tests throughout the next 3 months.

Iw oils also get it confirmed via PCR so you have a definite result

If traveling abroad you can test positive up to 90 days and could become stranded - a PCR confirmation would avoid this.

I was told that if a pupil has Covid they can show positive for 3 months on LFTs or PCRs, so does that mean they stop doing the routine LFTs as instructed by the school?

Because what would be the point if their ‘positive result’ is simply a result of them having had Covid the week before? It wouldn’t be acted upon as a new infection would it?!

And what if a child wakes up one day with some cold symptoms and a headache, after having had Covid six weeks ago, do they bother with a PCR? Because if it came back positive how would you know if it was a new positive or just left over particles from their previous Covid infection?

And so if LFTs and PCRs are pointless if taken within three months of a previous infection, do pupils have to isolate if they have cold symptoms ‘just in case’ it is Covid? So potentially isolating and missing school when they don’t actually need to because they don’t in fact have the virus but have no way of proving it?

It’s so confusing

IME, LFT positives wane very fast - if someone tests positive on them at all, it is only briefly and at the very height of the infection, because they are most reliable at times of very high viral load.

PCR tests can pick up virus for quite a long time after infection, so the 90 day rule seems sensible for those.

@Watapalava

Talipes

Ons data from March shows that 60% of the positive lft tests taken when kids went back were false positives

So hundreds of thousands of kids/contacts isolated for sod all

This was proven by pcr

60% false positive!

You have far fewer false positives when cases are higher. Cases are about 8 times higher than mid March so the proportion of false positives will be about 8 times lower.... about 8%.

The issue with not getting a pcr is the data, government wont realise how out of control Covid is at the moment. Numbers have not increased but they have its because people have been testing with the lft