Antibody tests - had one and it's negative and I now feel really upset

[FacingMyWaterloo]

I know this might sound ridiculous and I'm partially posting for you all to slap me round the face with a wet fish...

I work in central London and have a train/tube plus bus or walk commute. I'm defined as a key worker and though I have been able to do some work at home, I've also still had to go in, on and off.

In the middle of March, I had the coronavirus symptoms. 3 days after I came down with them, my partner (who is asthmatic, and works from home anyway) got the same symptoms as me but worse.

I need to do more work in town v soon so thought for peace of mind, we would have a private antibody test. We were absolutely convinced we had had it. His has come back positive and mine is negative and I feel totally devastated. I actually wish we hadn't had them done now. I did read there is more chance of a false negative and there are some people whose antibody levels don't pick up enough to test anyway.

Please talk some sense in me because for some reason I feel utterly miserable about it. Dp is of course delighted and now feels invincible yet he's the one who gets to stay at home anyway!

@oaktree55 you say “Reliable antibody testing is available privately at a cost for those willing to pay.” Who provides these and how to go about getting them done ?

People really need to know about negative predictive value and positive predictive value - especially if you are doing a 'screening' test like this - where we don't know the prevalence of antibodies in a population.

Even a highly specific test can still give a lot of false positives if a lot of the population is tested and they don't have the disease.

It's most likely a false negative. Or perhaps your partner had a false positive?! Not sure which specific test you did but my understanding is they are at best 70% accurate. In any case we don't yet know how long immunity (if any) lasts. If you did indeed catch it, which I would assume you did, then you didn't suffer a severe reaction anyway, so I wouldn't be too concerned. There is no evidence (yet) that even if you were to catch it twice that the symptoms would be more severe in a second or subsequent instance.

I can see why you're upset OP.

Best to brush it off. The companies playing on people's worries by offering inaccurate or misleading tests should be held to account

Hmm the gov are send tests out to 1k households so there must be accurate ones.
Nyc has tested as ha germany.

I think they just dont want us to know.
As if its 1-2% then our death rate is huge.
If 50% of us have had it they loked down really late.
Even 80% accuracy and 20% false neg would help identify people. How would they find out you can get it again unless some swab tested people come back positive again and that already happens.

Nyc has tested as ha germany

The word 'accuracy' is not really a good word to use here.

Positive predictive value and negative predictive value is better - especially if you are screening a general population.

If you do a test on a population that has had symptoms, then the PPV is better.

@chomalungma could you go through PPV and NPV please?

I don’t think these antibody tests are accurate

PPV and NPV

It's all to do with the maths, the population you are testing and how specific and sensitive a test is.

Let's say you have 10,000 people.
10% of the population have antibodies (this is just an example)

So 1000 people have antibodies.

A test could be 99% sensitive. So of those people who get tested.

990 test positive
10 test negative

9000 people don't have antibodies.

A test could be 99% specific

So 90 people test positive (false positive) and 8910 test negative

So altogether - you have 990 true positive / 1080 positive

So a 92% TPV - if you get a positive result, then you have a 92% chance of actually having antibodies

You can alter the values depending on how good the test is and how prevalent the antibodies are in the population being tested.