Researchers have figured out how to give an entire community a drug test using just a teaspoon of wastewater from a city’s sewer plant.

The test wouldn’t be used to finger any single person as a drug user. But it would help federal law enforcement and other agencies track the spread of dangerous drugs, like methamphetamines, across the country.

Oregon State University scientists tested 10 unnamed American cities for remnants of drugs, both legal and illegal, from wastewater streams. They were able to show that they could get a good snapshot of what people are taking.

“It’s a community urinalysis,” said Caleb Banta-Green, a University of Washington drug abuse researcher who was part of the Oregon State team. The scientists presented their results Tuesday at a meeting of the American Chemical Society in Boston.

Two federal agencies have taken samples from U.S. waterways to see if drug testing a whole city is doable, but they haven’t gotten as far as the Oregon researchers.

One of the early results of the new study showed big differences in methamphetamine use city to city. One urban area with a gambling industry had meth levels more than five times higher than other cities. Yet methamphetamine levels were virtually nonexistent in some smaller Midwestern locales, said Jennifer Field, the lead researcher and a professor of environmental toxicology at Oregon State.

The ingredient Americans consume and excrete the most was caffeine, Field said.

Cities in the experiment ranged from 17,000 to 600,000 in population, but Field declined to identify them, saying that could harm her relationship with the sewage plant operators.