Thread by @deeksj

TRACK-AND-TRACE: MANY PCR-ves WILL HAVE COVID-19

Government advice (gov.uk/guidance/nhs-t…)

“If your test is negative, you and other household members no longer need to self-isolate”

BUT chances of COVID could be 19% in this group
@petermbenglish @drhelmac @PHE_UK @drhelmac

DATA (*more needed*)

Of staff in UK hospital 43% with week off for COVID symptoms were RT-PCR+ve.

Sensitivity of RT-PCR - no clear data - stated to be as low as 70%.

Investigate worst case 43%, compare with 24% best case
Sens: 70% to 85%
Spec: 98% (makes little difference).

TECHNICAL STUFF #1 – NEGATIVE LIKELIHOOD RATIOS (LR-)

LR- = (100-sens)/spec

TECHNICAL STUFF #2 – PRE-TEST ODDS

Odds = prob/(1-prob)

When pre-test prob = 0.43, pre-test odds = 0.43/0.57 = 0.75

When pre-test prob = 0.24, pre-test odds = 0.24/0.76 = 0.32

TECHNICAL STUFF #3 – APPLY BAYES – worst case scenario

TECHNICAL STUFF #4 – APPLY BAYES – best case scenario

SUMMARY

RT-PCR does not have high sensitivity – high chance of false negatives.

Prevalence of COVID-19 in people with COVID like symptoms is high.

NEGATIVE tests do not RULE OUT COVID-19.

Probability of COVID-19 in RT-PCR-ve far too high to treat them as uninfected.