Colorado just enacted the nation’s first law banning arrests based solely on the results of colorimetric drug tests – a field test widely used by law enforcement across the country.

The tests are popular because they’re cheap, portable and can screen for drugs in mere minutes. It’s just not feasible to send all suspected drug samples to state laboratories, which would be far more expensive and could take days or weeks to return results.

But these inexpensive tests also lead to false positives at alarming rates, researchers from the University of Pennsylvania found.

While the actual error rate nationwide is unknown, previous studies by manufacturers have put it around 4%. But the UPenn researchers believe the actual rate is much higher, from 15% to 38%. And a study by the New York City Department of Investigation showed test error rates from 79% to 91% in some correctional settings.