A Charleston hospital security guard found a small packet of brown powder in Marshall's shorts pocket. Police thought it was heroin.

The 26-year-old Marshall insisted it wasn't. It's just my wife's asthma pills, he said. They must have gotten mushy in the washing machine.

Yeah, right, the police replied. We've heard that one before.

Except this story panned out.

State chemists proved him right. Charges were dropped Monday and Marshall went home.

``I sat six weeks in jail waiting for the information to get back and clear my name,'' the Charleston County man said.

Melisa Gay, his attorney, doesn't blame the police -- she blames the system. The police, she said, did everything they were supposed to do.

After the security guard called police, the officers did a field drug test. They dropped samples of the powder into a tiny plastic tube filled with chemicals and shook it up.

The fluid changed color. That usually means heroin.

The powder weighed 3 grams. Under S.C. law, that means possession of heroin with intent to distribute, and he was arrested. Bond was set out of Marshall's reach, at $100,000, because he was already out on bond for two assault charges.

So he sat in the Charleston County Detention Center, waiting for justice.

He could have been there for six, eight, 10 months.

That's often how long it takes the S.C. Law Enforcement Division to test confiscated drugs.

But Gay called the solicitor's office and insisted her client was innocent. So Assistant Solicitor Amie Clifford -- who usually laughs off such claims -- pressed SLED to test the stuff.

Turns out, the brown powder was guaiphenesin, an over-the-counter expectorant found in Robitussin and Bronkaid.

And Marshall was vindicated.

No one knows how the field test could have confused asthma medicine with heroin.

It's possible that there were traces of heroin in the packet with the guaiphenesin, Clifford said, although there's no proof of that.

Results in field tests can be affected by unexpected residues mixed with the substance, police said, including chemicals in laundry detergent.

``Field tests are not 100 percent accurate,'' said Charleston police Lt. Melvin Cumbee, whose department arrested Marshall.

Courts acknowledge the unreliability of field tests, Gay says. They are not considered proof worthy of conviction; they must be substantiated by a licensed chemist.

Nevertheless, positive field tests are considered probable cause for an arrest, a fact that frustrates Gay.

``You can have harmless stuff in your pocket one moment and the next minute you could be in jail,'' said Gay, who anticipates no further legal action.

``It is sickening that it can happen. And it can happen to anybody.''

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