Thread by @petermbenglish

1/ I couldn't find a link to the study in this BBC News report, but the report rehashes what we've known for a long time.

bbc.co.uk/news/health-58…

We know:

2/ PCR tests on throat swabs, done with good technique at the right time are only about 70% sensitive: 3/10 positive cases are missed.

PCR tests done late (eg a day or 2 after a positive lateral flow test) is even less sensitive.

Lateral flow tests are even less sensitive.

3/ Both types of test are extremely specific: fewer than 1% of people who do not have Covid-19 will get a false positive result.

4/ (Especially if you discount late positive results - RNA from non-viable, "dead" viruses can persist for weeks, but infectiousness is 7-10 days, max.)

The probability you're genuinely infected is >99% after either type of test, if positive.

5/ A late PCR test might reduce this slightly, but the probability will only come down a fraction, likely still ~99%.

So, while "confirmatory testing" may help eg by identifying the genomics of the virus, it should NEVER be used to say the initial test result was incorrect.

6/ So it should never change decisions to self isolate or take other precautions.

We knew this 18 months ago, and any advice that quarantine in somebody who had tested positive could be curtailed could be was never driven by public health advice or science.

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