False positives?!

[TryingNotToPanicOverCovid]

Are they a thing? Its highly unlikely isn't it? I know how it might not pick up a covid case if not taken properly or a case can be missed. But is a positive from a test centre always a positive?

I've had a positive covid test but the test of my family have all since tested negative. I was one of those who cant work out how I got it (only walking outdoors and tesco delivering) but aware its super transmissible now. But if so how has my family not got it?

I have had a cough for definite. And a tight chest (but I have asthma si starting to worry Ive imagined it all . I have cv th8ngs and was so expecting to be hospitalised if I got it which I haven't. Oxygen has constantly been fine...

Id like to think I have antibodies and stop avoiding shops etc now.. . But now Im doubting myself!

But if so how has my family not got it?

Maybe they have but are asymptomatic & have false negatives.

2 X Colleagues at work, their (separate) families had it but they both tested negative. They had no symptoms but couldn’t understand how they didn’t catch it. One of their DP’s had no symptoms but tested positive.
Both colleagues have tested positive for antibodies so they did actually have it.

3 false negatives?

I really like to understand (have followed a lot the data threads) and don't really "get" it!

My 16 year old DS has it and none of us have got it. I’ve done 2 PCR tests and lots of lateral flow tests (I’m not at work due to isolating but still need to do them) and they have all been negative. He literally hadn’t left the house for 2 weeks prior to his positive test. His only symptom was that he could smell something odd. Whereas I work in a school and my youngest DS also attends so it would make more sense for us to get it?
I’m assuming we are somehow immune and he has just got it from us?

That's so weird isn't it. I must have caught it walking too near someone outdoors yet my children who are surgically attached to me (or so it feels) havent got it.

Mayve they've had it asymptomatically in the past. Or it doesn't get everyone. Or something....

It's possible to get a false positive as no test is 100%, but obviously you need to follow the advice assuming you do have it.

if LFT test you are supposed to confirm with a pcr as false positives happen?

It was one at a proper test centre so I assume that's pcr? I was aware the lateral flow ones werent accurate in that they missed a lot of people.

Can I be pretty sure I've had it then?

Could your family have had it before you? If you caught it at the end of their asymptomatic illness could they then test negative as they're better? It definitely doesn't seem to affect everyone, which is odd but good.

I guess that's possible if they were asymptomatic.

I also heard that covid can show up on a test for upto 90 days so I assumed one of the 3 of them might show up.

Or mayve I had it a while ago. Weve not been anywhere since xmas so I guess more likely I had it when the kids were in school but its a mystery.

I wondered about paying for an antibody test but we're pretty broke - and I know they dont always show either!

False positives on the PCR test are definitely a thing. No test is 100% accurate and the PCR test is quite sensitive so not too prone to false negatives. We don't know how many false positives there are but SAGE says it is between 0.8 and 4% of all tests. When rates were low in the summer there were not enough positives, false or true, for 4% to be correct.

Say, for the sake of argument that the rate of false positives is 2% (more-or-less the halfway point in SAGE's estimate). Today's Gov.UK dashboard has PCR testing 'capacity' as 770,305. If that's the number of tests carried out, at 2% there would be 15,406 false positives. The dashboard gives us 20,634 positives for today. Of course, we don't know how many of the tests and the positives were on the same day. I don't think we know how many of the positives came of each type of test.

Lateral flow tests, however, probably give a lower false positive rate because they are less sensitive. The downside to that is they give a higher false negative rate.

There was a cruise ship where a passenger tested positive and the ship returned to port, the cruise was cancelled and all the passengers sent home. The passenger was tested twice more, once using the same sample. Both tests were negative.

Oh gosh wow so it could be. Eek.

I've decided to pay the £49 for an antibody test...