GREENVILLE -- Sometimes people fail a drug test, and have no idea why.

They don’t take illegal drugs.

But sometimes, substances that seem perfectly harmless can affect the results of a drug screening. That’s why it’s important to understand the process and what can affect it. There are actually two levels of drug tests.

The most sensitive is something called gas chromatography mass spectrum.

A test called immuno assay is actually more of a screening tool.

It can be very good...but not as sensitive as the g-c-m-s.

The immuno assay is very accurate for cocaine or marijuana, but there's one substance that could trigger a false positive.

The active ingredient in Sudafed, pseudoephedrine, could trigger a false positive for methamphetamine.

An adult drug test has cut off levels to avoid false positives, but it’s different for children. There is no cut off. So eating a poppy seed bagel or muffin could lead to a reading for opiates.

"Yes it's possible,” says Andre Canipe, operations manager for Accurate Diagnostics.” The poppy is used to produce the opiate class of drugs. “Once you eliminate these environmental exposures or those cutoff levels that are set,” he says, “absolutely, it's possible they could show up as that. But any federal workplace or any kind of legal drug testing, no absolutely not."

South Carolina has a very stringent and detailed drug testing law. Any positives that show up in a screening are re-tested using the more sensitive test just to be sure of the results.

By the way, hair testing is used when a drug history going back 90 days is wanted. It’s the test used for children. Otherwise, urine testing is used.