"The Curious Case of the 1% Positive Rate Baseline in #Ireland"

A #COVID19 thread.

The HPSC and HSE provides us with this chart for "Positive Rate (%) Previous 7 Days" on their dashboard. Link: https://covid19ireland-geohive.hub.arcgis.com/pages/hospitals-icu--testing

Positive Rate (%) is going up. Let's examine closer.

1/14

Dr Cillian de Gascun, the chairman of the National Public Health Expert Team ( #NPHET) stated in a detailed thread last week that "false positive" test results occur at a rate in the region of 1-3%.

Tweet Link: https://twitter.com/CillianDeGascun/status/1305252820313141249

2/14

Let's mark the False Positive Range (1-3%) on the chart, along with highlighting 1% as a baseline for future reference.

Notice anything interesting?

3/14

From the start of August the Positive Rate rose above the 1% Positive Rate Baseline and has stayed above it ever since. It has also stayed exclusively within the False Positive Rate of 1-3%.

"So what?" you may ask. Cases are going up after all. Let's continue...

4/14

The really interesting section of this chart happens below the 1% Positive Rate Baseline. Let's mark it clearly.

So what makes this period worth examining?

5/14

We've been told that "false positives" occur at a rate in the region of 1-3% so as a result we should never have Positive Rate (%) go below 1%.

So how is it possible that we saw the Positive Rate (%) reported below 1% for an almost two month period?

6/14

The #PCR tests used to detect #COVID19 run at a predefined number of cycles. Dr Cillian de Gascun stated that these tests usually run for 35-45 cycles. This is the number of cycles that return the False Positive Rate of 1-3%.

Tweet link: https://twitter.com/CillianDeGascun/status/1305251821829656585

7/14

If the #PCR tests are run at a lesser number of cycles they pick up less "false positives".

Is it possible that the PCR tests for #COVID19 in #Ireland were run at a lower cycle level at the start of the pandemic, but got readjusted to the current 35-45 cycles? If so, why?

8/14

Also of huge importance is the current "false positive" rate.

Since the current reported Positive Rate (%) falls between the False Positive 1-3% range, is it possible that the majority of recent cases are "false positives"?

9/14

That may explain the rise of #COVID19 cases in #Ireland not being matched by a proportional increase in deaths.

10/14

One theory as to why Positive Rate (%) was reported below the 1% Positive Rate Baseline may be that stricter hospital-based testing was used during the height of the pandemic, so there was less chance of "false positives".

11/14

However if that is true, why are positive cases currently being reported with the same level of urgency as during the height of the pandemic if there is a greater chance of "false positives"?

If the testing process has changed substantially the public needs to know this.

12/14

Given the dramatic impact on people's lives that the Irish Government's, #NPHET's and HSE's decisions have made, #COVID19 testing processes are an extremely important area that there must be full transparency on.

13/14

We need #PCR test cycle thresholds and "false positive" rates included wherever they have input to decisions.

Here is the full graph again. Please share if you found this helpful!

/thread end