COVID test positivity is not a meaningful statistic because not all negatives are reported


Updated with a link to Florida’s reporting rule.

The percentage of positive COVID tests has increased dramatically in Alachua County and in Florida, and we now have evidence that this percentage is misleading at best.

The percentage of positive tests has been used throughout the pandemic as a measurement of community spread, with most public health authorities agreeing that it is best to keep that rate below 10% (with 5% being even better).

Alachua County’s positivity rate hovered around 1% from May 9 to June 10, and it suddenly spiked on June 11 when the migrant farmworker cluster (with 90% positivity) was reported.

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Also around that time, the University of Florida stepped up its employee testing in preparation for their return to campus, and we also know that the university and its hospitals started testing athletes and hospital admissions, at least. Other employers are probably also testing employees. A quick scan of the labs reporting results to the state (starting on page 24) shows that a number of labs and hospitals are reporting 100% positive tests–i.e., they are not reporting the negatives.

It looks like North Florida Regional and Shands are reporting at least some negatives, but we now know that UF is not reporting negatives in employee testing to the state. By law, all test results by physicians, hospitals, and laboratories are required to be reported to the state, but the list of labs reporting 100% positives indicates that’s not happening–or perhaps tests performed by an employer are not covered by the order.

We know that not all of the local negatives are being reported because UF reports the results of its employee testing here; we log that number on a spreadsheet when it changes, and the number of total tests increased by 3,527 from July 4 to today (with only one positive test). Over that same period, the state reported an increase of 1,423 tests (with 195 positive tests). [Editor’s note: The total number of tests reported by UF mysteriously dropped by 3,186 a day after this article was published.]

That means that the official positivity rate for those three days was 13.7%, but when you add the unreported negatives from UF, it becomes 3.9% (and that’s only adding the negatives we know about; there are almost certainly others). That paints a very different picture of community spread.

If negative tests aren’t reported, they aren’t added to the total number of tests. That makes the reported positivity rate higher than the actual positive percentage of all tests performed. This can easily be seen when you look at the hospitals reporting 100% positive tests. If they reported 50 positives with 100% positive tests, for example, the number of tests reported to the state is 50. If they did 500 tests to get those 50 positives, 450 tests were not reported to the state and were not added to the total number of tests reported by the state.