The white powder in Karlos Cashe's car was drywall that he used as a handyman in Florida. But when a police officer pulled him over in March for driving without headlights and tested the powder with a field drug test, it came back positive for cocaine.

WFTV - wftv.com

Cashe, who had a drug-related conviction from 2015, was arrested and denied bond due to his being on probation. He spent 90 days in jail before a laboratory analysis contradicted the field drug test last week. Now that all charges have been dropped, he's hoping his case will shed light on the shortcomings of these roadside tests.

"I sat there 90 days knowing I was innocent," Cashe told WFTV. "This is what I want to stop. I don't want this to happen to anybody else."

Wikimedia - wikimedia.org

But odds are it will happen again. Police around the U.S. commonly use inexpensive roadside tests to expedite drug possession convictions, as a 2016 investigation from ProPublica revealed. The news organization estimated that "at least 100,000 people nationwide plead guilty to drug-possession charges that rely on field-test results as evidence" each year.

"At that volume, even the most modest of error rates could produce thousands of wrongful convictions." — ProPublica

There are numerous factors that can lead to a false positive or cause a police officer to misinterpret the results of field drug tests. For the most common roadside drug test, which changes colors when they interact with certain illegal drugs, there are at least 80 legal substances that also cause the test to change colors. Acne medications can cause a false positive, for example, and even the temperature outside has been shown to affect the results.

"Drug tests, generally like any other forensic discipline, have shortcomings," Jolene Foman, a staff attorney at the Drug Policy Alliance, told ATTN:. "There are all sorts of things that can botch these tests," especially when you use roadside versions that are less controlled and can be contaminated.

But because the subsequent laboratory analysis can take months, there's pressure on suspects to plead guilty even when they know they're innocent.

ProPublica - propublica.org

ProPublica looked at 300 cases where people were wrongfully convicted of drug-related crimes based on field drug test results. In more than half (57 percent) of the cases examined, the suspect pled guilty within a week of their arrest—only to be later proven innocent.