We’ve been hearing for weeks about the crush of patients crowding intensive care units in hospitals as California copes with the worst phase of the coronavirus pandemic. But when the Golden State’s official calculation for statewide ICU capacity dropped to 0% late last week, it raised a question that anyone with a basic understanding of math would find perplexing:

How could something be at “zero percent” — for example, ICU beds — when some are still available?

Turns out, as with so many metrics tracking the COVID-19 outbreak, the state’s definition of “capacity” is far from simple.

With Gov. Gavin Newsom’s office desperately trying to keep hospitals from being overwhelmed, the state on Dec. 3 linked its latest stay-at-home orders to ICU capacity. It broke California into five regions and announced that new restrictions would be triggered when ICU capacity dropped below 15%.

Southern California was quickly under the new order. And since Dec. 17, four regions, home to 98% of the state’s population, have been under the Regional Stay Home Order. The only region that has not dropped below 15% is the sparsely populated Northern California region, which had 29% ICU availability as of Monday.

Alarms have been going off since two of the regions — sprawling Southern California and the San Joaquin Valley — dropped nearly two weeks ago to 0% on the ICU capacity meter. The Bay Area region is at 9.5%, and the Greater Sacramento region is at 16.6%.

But how do those percentages, ranging from zero to nearly 30, add up to a statewide ICU capacity of 0%?

To add to the confusion, the California Department of Public Health also reports that as of Sunday there were 1,385 available ICU beds in the state.

We’ve been asking the CDPH to clarify how these seemingly contradictory figures add up, and late last week they provided an email with an answer: The ICU capacity measure “is standardized to reflect effective capacity in ICUs by looking at the percentage of COVID-19 positive patients in the ICU,” the department wrote.

So that would mean each region has an ICU capacity that is “adjusted” to account for the intense demand for COVID-19 care.

CDPH says it calculates the adjusted ICU capacity based on the proportion of ICU patients who have COVID-19. “If a region is utilizing more than 30% of its ICU beds for COVID-19 positive patients, then its available ICU capacity is adjusted downward by 0.5% for each 1% over the 30% threshold,” according to the CDPH office of public affairs.

So that mind-bending formula explains how the state has 0% capacity with more than 1,300 ICU beds still available — and maybe why an explanation has been so hard to come by.

CDPH has still not responded to a request for the actual and adjusted ICU capacity measures for each of the state’s five regions. But figures reported by Orange County’s public health department provided a hint of how it works. As of Monday, the county reported that the hard-hit Southern California region’s ICU bed availability was 0.0%, adjusted, and 11.1% unadjusted. That suggests Southern California’s ICU’s have more than 50% COVID patients, according to the state’s formula for adjusting capacity.

San Bernardino, one of the hardest-hit counties in Southern California, provides some additional details that lend insight on the ICU crisis. The chart below from the county’s coronavirus data dashboard shows the total number of ICU beds has risen slightly since early October, while COVID patients in the ICU have shot up. Now, COVID patients appear to make up more than half of all patients in the ICU, but some beds are still available.

Screenshot from San Bernardino County Public Health Department's COVID dashboard, showing total ICU beds, COVID patients in the ICU, and other ICU patients.

CDPH said it adjusts the capacity measure “to preserve the capacity of the ICU to also treat non-COVID-19 conditions.”

Adjusted or unadjusted, health experts say ICU capacity is a highly fluid number that also factors in the availability of ICU nurses, necessary equipment, and suitable space — not simply beds. Ultimately, experts say, capacity can be increased, but gauging capacity is an essential tool to monitor how much we’re straining the system.