Federal government estimates of opioid overdose deaths have been rising year after horrifying year. But the crisis is even worse than it looks: A new study reveals that the government has been undercounting opioid overdose deaths by 20 percent to 35 percent.

To estimate national trends in opioid overdose, the federal Centers for Disease Control and Prevention aggregates data from more than 3,000 coroner’s offices around the country. Coroners function independently and vary widely in their available resources and reporting practices. University of Virginia professor Christopher Ruhm noticed that many coroners did not record a specific drug when documenting a fatal overdose, implying that many opioid overdose deaths were not being counted in official figures.

By studying the records of coroners who did record specific drugs for overdose deaths, Ruhm was able to impute a corrected count of opioid overdoses. According to Ruhm’s research, if all coroners accurately reported opioid overdose deaths, official counts would be substantially higher. For example, the CDC figure for 2016 was 42,249 opioid overdose deaths nationwide, but with accurate data the count would have been 49,562, Ruhm said.

The CDC cannot control local coroners, so it’s up to states and counties to improve overdose reporting practices. Although some of them have been doing so, the political incentives are not well-aligned. For example, if a state or county spends money on better recording of overdose deaths it will be “rewarded” by looking as if its opioid problem is getting worse.

More important, Ruhm’s research shows that the severity of the opioid epidemic is being underestimated. This makes it all the more urgent for the president and Congress to take significant steps to curb the worst public health disaster in decades.