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Testing trends: 5% positive is great, but 5% false positive is not

Posted at 7:26 PM, Apr 21, 2020
and last updated 2020-04-21 22:09:15-04

Utah’s testing as of late April is the envy of the nation. We rank sixth in the number of tests per capita, and 43rd in the number of overall deaths, despite being the 30th state by population.

All that testing has consistently yielded about 5% positive results. Compare that to the two states with the most tests per capita: Rhode Island has a 14% positive rate and New York has a 39% positive rate.

Now Utah’s ending its run hovering around 5% or just above because the positive rate is approaching 4.5%.

In the meantime, there’s a lot of talk about antibody tests nationally and in Utah.

Antibody tests are being conducted by respected medical organizations including ARUP labs at the University of Utah, but ARUP makes clear they are performing the tests for the purposes of research and not to inform individual patients about their immunity.

There’s a good reason for ARUP’s policy. Antibody tests currently can return false positives at rates around 5% according to recent reporting in the New York Times. That would mean a person would be foolish to rely on such a test for reassurance of immunity.

Imagine if you work at a small company with 20 employees, all of whom are tested. If five percent of the population has had the virus (a hypothetical) and the test returns a five percent false positive rate, two employees might get positive results with neither knowing if they really have it.

Extrapolate the same example to an entire city, and you’d have tens of thousands of people with false positive results.