The accuracy of drug tests once depended on proper chain of custody and a competent laboratory. Employers needed a cheaper, easier alternative, and oral swab testing has given them one. The concern is whether oral fluid and on-the-spot testing materials are as accurate as urine tests.
The accuracy of drug testing methods is determined over a documented period of use, with large representative samples and thorough follow-up with test subjects. Oral swabs are a commercial product, often used and read on the spot, so much of the data required to determine overall accuracy is not recorded systematically.
Oral fluids are similar to plasma in the way they carry drug traces. But there are more ways an oral test can be compromised: a recent meal; changes in fluid concentrations sometimes caused by drug use; other legal drugs; or the use of a detox agent by the person providing the sample.
Drug evacuation times for saliva differ based on the drugs ingested and the method of ingestion. Marijuana might evacuate within 14 hours but show up strongly in the first hour because the drug is taken by mouth and saliva absorbs some of it. Cocaine, amphetamines and opiates can stay in the saliva for up to three days after ingestion.
Other factors impinging on accuracy include false positives, which can be caused by everything from ibuprofen to poppy seed buns. The brief duration of drug traces in saliva, and the wide availability of detox formulas, mean the test is easy to beat. Saliva testing is still not accepted by the federal government for its employees.