Bucks prosecutors and police, however, hail the tests as a way to ensure that cases quickly reach the trial stage. Longer wait times for official lab results at the county lab have delayed preliminary hearings for some cases and therefore their subsequent trials.
Some cases have dragged on so long that judges had to reduce bail for defendants, freeing them.
"These field tests serve the greater good by speeding up the prosecutions," said Matt Weintraub, chief of prosecution for Bucks County. "One false positive is too many. But I feel like we have too many safeguards in place that I'm confident that no defendant is going to be convicted as a result of a false positive drug field test."
Weintraub said prosecutors don't charge people based on drug kit results alone, factoring in other variables - such as whether someone is also carrying a large amount of cash and has suspected substances separated in several bags as if they will be sold.
But in other jurisdictions, false positives have landed innocent people in jail. Last fall, a state trooper pulled over a couple on I-78 in Lehigh County for driving 5 m.p.h. above the speed limit and veering too close to the next lane.
The smell of marijuana led to a search of their rented Mercedes-Benz, which contained four pounds of a white powdery substance wrapped in plastic bags. The woman who was driving, Annadel Cruz, said it was homemade soap. A field test determined it was cocaine.
The New York City couple was jailed on felony drug charges. But three weeks later, lab results found that the substance was indeed soap. The couple was freed.
"Just one false positive is reason enough not to do it," said Robert Goldman, a Bucks County-based defense attorney and a former prosecutor who represented the woman in the Lehigh County case.
Other such cases include a man who sued New York City's police department last year after a field test incorrectly determined that jolly ranchers in his possession were methamphetamine. And in 2011, a 66-year-old Minnesota woman spent 12 days in a Canadian jail after border authorities field tested a bottle of motor oil. The test incorrectly indicated the presence of heroin.
Call for a moratorium
In 2008, a report backed by the Marijuana Policy Project called for a moratorium on such drug tests and more oversight because of false positives.
The report's introduction cites Janet Lee, a Bryn Mawr student who spent three weeks in a Philadelphia jail in 2003. A field test had incorrectly found that a flour-filled condom in her luggage, a gag from friends, contained opiates and methamphetamine. She later sued, and the case was settled.
"If anybody has any doubt," said Goldman, the lawyer who represented Cruz in Lehigh County, "ask yourself how you would feel after a night in jail. Putting an innocent person in jail for one day is more important than a backlog of lab tests."
The test kits are often comprised of pouches containing chemicals. A sample of the suspected substance is placed inside, and if the test is positive, the liquid will turn a certain color. Officers receive training to use them.
In 2008, the National Forensic Science Technology Center, a Florida-based nonprofit, reviewed the four major brands of test kits and found them to be appropriate for use at the start of drug investigations. However, the study, which had the backing of the Justice Department, noted that the tests are subject to false positives and should be backed up by lab tests at the trial level.
Morrisville police Chief George McClay, a former lieutenant in Philadelphia who arrived in February, said he used the kits for decades in the city and never encountered a false positive.
"When I found out our officers [in Morrisville] were continuing cases time after time and defendants were being released on bail, it was time to make a simple move" [and use the tests, McClay said.
Richard Long, executive director of the Pennsylvania District Attorneys Association, said the benefit to the criminal justice system outweighs the rare likelihood of a botched test.
"I'm not going to say it doesn't happen," he said. "But if it happens, that doesn't mean it shouldn't be done."