One morning in September 2017, Judge Christopher Plourd opened an unusual hearing at the Imperial County Superior Courthouse, a half-hour north of the California-Mexico border. It involved three illegal drug possession cases that were unrelated to one another.

Each of the cases had relied on the results of chemical field test kits used by corrections officers at nearby state prisons. The kits indicated crumbs and shreds of paper that guards found on the inmates contained heroin and amphetamine. But a state forensic laboratory later analyzed the debris utilizing a far more reliable test and found no trace of illegal drugs. The defendants were factually innocent.

Rather than simply close the cases, defense attorneys asked the court to determine whether the NIK Public Safety brand field tests used in California’s prisons were too unreliable to show to grand jurors. In effect, they put on trial the evidence most commonly used to secure convictions in drug cases in the U.S.

Plourd ruled in early 2018 that the test kit “does not meet a scientific admissibility standard” and therefore “does not support the grand jury indictment.”

In other words, the tests were guilty.

The Imperial County cases are believed to be the first time a judge blocked field tests from contributing to indictments. In the years since, defendants and inmates in multiple states have scored additional legal victories against agencies using the kits and the companies selling them.

“For years, these tests have had this unjustified scientific veneer,” said Des Walsh, founder of the Roadside Drug Test Innocence Alliance, which advocates for the use of more accurate testing technology. “Finally, we believe the tide is turning with this dawning awareness of the unacceptably high rate of false positives.”

In a 2016 series of stories, ProPublica documented law enforcement’s widespread use of field tests to make arrests and secure convictions despite serious flaws. No government agency regulates their use. The officers who perform the tests to make arrests on the street often have little or no training in their use.

Since then, the new court rulings have contributed to a growing movement to change the way drug cases are prosecuted in America. Courts across the country have long known that field tests are error prone and require forensic laboratories to confirm the results for jury trials. However, nearly all drug convictions in the U.S. come by plea deals during initial hearings, where chemical kits are the primary evidence of guilt.

Courts have overturned 131 drug convictions in the past 10 years after laboratory analysis determined the alleged drugs were legal substances, according to a database maintained by the National Registry of Exonerations. A large majority of those wrongful convictions originated in Harris County, Texas, where the crime lab analyzed its backlog of suspected drugs from closed cases and discovered the evidence in hundreds of convictions did not contain drugs. The defendants in those cases had pleaded guilty at preliminary hearings.

The tests are small plastic pouches holding vials of chemicals. They’re cheap, roughly $2 apiece, and easy to use. Officers open the pouch and add the substance to be tested. The tests are designed to produce specific colors when mixed with drugs like heroin, cocaine or methamphetamine. But dozens of items, including foods and household cleaners, trigger similar reactions.

During the Imperial County hearing, an executive at the Safariland Group, the nation’s largest field test manufacturer, testified the company keeps a list of more than 50 legal substances that cause positive results. Court records show chocolate sometimes turns the liquid a similar shade of green as heroin in the NIK kits.

Safariland Group did not respond to a request for comment.

More evidence of the tests’ inaccuracy came in October 2021, when former inmates filed a class-action lawsuit against the Massachusetts Department of Correction. The prisons used test kits on all incoming mail, including letters from attorneys. When correspondence tested positive, inmates were sometimes put in solitary confinement and lost eligibility for parole. The lawsuit alleged that the prison system’s use of field tests violated the inmates’ right to due process.

Court records show that between August 2019 and August 2020, lab analysis found that 38% of the inmate mail that tested positive did not contain the alleged drug. Shortly after the inmates filed their lawsuit, Suffolk County Superior Court Judge Brian David ordered the Correction Department to immediately stop using the chemical kits until the litigation was finished.

In the order, David characterized the NARK II brand kits used in Massachusetts’ prisons as “arbitrary and unlawful guesswork.”

The inmates are also suing Sirchie Acquisition Co., manufacturer of the NARK II kits, and Premier Biotech, a retailer that sells them, in federal court for negligence, alleging the companies misrepresented the kits’ risk of false positives and provided inaccurate instructions to the state prisons. In September, a federal judge ruled that field test sellers can potentially be held liable for harm caused by erroneous results. Both of the lawsuits are ongoing.

Sirchie did not respond to ProPublica’s request for comment. Sirchie, Premier Biotech and the Massachusetts Correction Department have denied the inmates’ claims in court records.

Compounding field tests’ inherent flaws, police officers and prison guards rarely understand how the kits work, according to court records and interviews.

During the hearing in Imperial County, multiple guards testified about the training they received on the field tests and how they described the results to grand jurors. David Eustaquio, an officer with the California Department of Corrections and Rehabilitation, told the court he had used the chemical kits more than 200 times during his career, according to transcripts. He said he’d never had to explain the results beyond saying the color change meant the test was positive for an illegal drug.

“Do you know what the accuracy rate is for these NIK tests?” Kelly Jafine, an Imperial County deputy public defender, asked Eustaquio.

“No, I do not,” he said.

Jafine then asked if the prison had taught him about false positive results during training on the chemical kits.

“No,” Eustaquio answered, “I was not.”