Ask LAist: Are The Lower COVID-19 Case Numbers Due To Less Testing?

A health care worker hands a patient a COVID-19 testing kit at a drive-thru site at The Forum. (Courtesy of the County of Los Angeles)

We're answering your questions about COVID-19 and vaccines here in Southern California.

A reader wrote in and told us about something they're worried about:

"I wanted to ask if you could research this a little. I'm pretty concerned that the COVID stats for infections are starting to degrade because most testing centers are being converted to vaccination centers ... if you're not testing, how can you really know that your infection rate is going down? Doesn't that skew the statistics? Is your infection rate only going down because your volume of testing is going down?"

Let's break this question into a few parts.

First: In January, Dr. Christina Ghaly, director of the Los Angeles County Department of Health Services, acknowledged that switching a big testing site like Dodger Stadium to a vaccination site might create some "testing gaps" that would need to be filled.

But by February, L.A. County Public Health Chief Science Officer Dr. Paul Simon told reporters, "we feel like we've addressed that and we've been able to ramp back up testing."

And while Dodger Stadium was a big testing site, there are still many more places you can get tested for COVID-19 — including from home.

That said, testing still isn't back to the level it was earlier this year.

We know this because the county publishes a dashboard that shows what's going on at sites run by the city and county.

And on that dashboard, you can see that the number of completed tests at those sites — in the top chart, in blue — has gone down.

The County of Los Angeles publishes a dashboard showing how COVID-19 testing is going at sites run by the city, county, and state. (Screenshot of a County of Los Angeles document)

The peak was one week at the beginning of the year — Jan. 3 to Jan. 9 — when the sites administered almost 300,000 tests.

Compare that with earlier this month — March 7 to March 13 — when those sites completed just over 41,000 tests in a week.

LOWER DEMAND

On that same dashboard, you can also see unused testing capacity (in yellow).

Take that same week from earlier this month. The dashboard shows there were another 180,000 available tests that went unused, either because someone didn't show up or because an appointment was never scheduled.

I asked the L.A. County Department of Health Services why it thinks demand for tests is so much lower now.

"Though we can't draw any specific conclusions, the decrease in demand for testing may be the result of a variety of factors, such as a decline in COVID-19 cases and community transmission, as well as the increased vaccination effort," the department told me.

So those circumstances might explain why fewer people are seeking tests now.

POSITIVE TESTS

There is another way to compare tests and cases over time, though, and that's by looking at how many tests taken come back positive (also known as the test positivity rate).

"In California, in this past week, we've done over a million tests and only 1.6% of them were positive, which is a low number we feel good about," USC virology professor Paula Cannon told my colleagues at KPCC's Take Two earlier this week.

In L.A. County, that rate bounced around 20% in January. Now, it's less than 2%.

You can monitor the seven-day daily average testing positivity rate by visiting the county's COVID-19 "surveillance" dashboard and clicking "Table: Tests by Date."

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