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Test positivity rate: explainer and analysis
Test positivity in SA
has decreased this past week coming down from record highs. This is good news (we'll hold onto any good news), but not an indication that we are past the worst yet since cases are still high... Thread 
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Test positivity in SA
has decreased this past week coming down from record highs. This is good news (we'll hold onto any good news), but not an indication that we are past the worst yet since cases are still high... Thread 1/n
Test positivity is useful in analysing if any increase or decrease in confirmed cases is real. In test positivity has been high throughout the pandemic, so we cannot just look at case numbers alone but need to consider cases and tests or test positivity in combination... 2/n
test positivity has been high throughout the pandemic, so we cannot just look at case numbers alone but need to consider cases and tests or test positivity in combination... 2/n
I started tracking and sharing test positivity in SA since April 2020. Mathematically, test positivity = cases divided by tests, expressed as a percentage. So if test positivity decreases either cases have decreased or tests have increased at a higher rate and vice versa... 3/n
High test positivity is an indication of low testing, many undetected cases, and widespread community transmission.
The WHO recommends this measure be kept below 5%, while in SA, since testing is not wide enough, we aim to keep it below 10% as a guideline... 4/n
The WHO recommends this measure be kept below 5%, while in SA, since testing is not wide enough, we aim to keep it below 10% as a guideline... 4/n
I started tracking and sharing test positivity in SA since April 2020. Throughout the pandemic in testing has not kept up with the increased rate of transmission of the virus (even when transmission slowed after 1st wave, testing decreased too), up until last week... 5/n
testing has not kept up with the increased rate of transmission of the virus (even when transmission slowed after 1st wave, testing decreased too), up until last week... 5/n
So previously we could just look at the test positivity to decrease as an indication that the infection rates have decreased (since only the numerator was increasing/decreasing at a higher rate than the denominator)... 6/n
Of course, transmission and testing rates vary in different regions & provinces. Test positivity has varied in timing and rate between the provinces too. Unfortunately daily test numbers per province aren't publicly available so we have to analyse provinces retrospectively... 7/n
Since last week, testing in has increased significantly. So while cases have increased tests have increased at a higher rate since last week, so test positivity lower.
It is still a good indication since test positivity could have still increased with increased testing... 8/n
has increased significantly. So while cases have increased tests have increased at a higher rate since last week, so test positivity lower. It is still a good indication since test positivity could have still increased with increased testing... 8/n
We are testing much more, and finding more negatives. Cases still very very high, but for the first time really testing has increased quicker than cases, hence lower test positivity. So doesn't actually mean transmission has slowed... 9/n
Summary TL;DR: look at both case numbers and test positivity:
• cases and test +ve increase = very bad
• test +ve increase = bad
• cases increase and tst +ve decrease = good but needs more analysis
• cases and test +ve decrease = very good
Hope this is helpful!
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• cases and test +ve increase = very bad
• test +ve increase = bad
• cases increase and tst +ve decrease = good but needs more analysis
• cases and test +ve decrease = very good
Hope this is helpful!
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