A number of adulterants cause negatives on the commonly used EMIT test, but not necessarily other tests. In an emergency, you might fool an EMIT test by adding a few drops of Visine to the sample. Other common adulterants include detergent, bleach, salt, and the cleaning solution glutaraldahyde, but most of these are easily detected by smell or visual inspection. Many drug testing companies claim to test for adulterants, though it isn't clear how carefully. Many drug testing aids are advertised with money-back guarantees if you test positive.
Vendors often exaggerate product reliability and ignore customer complaints.
In some situations, including accidents and roadside sobriety checks, blood tests may be used. Blood tests generally register positive for just a few hours after smoking, though heavy chronic smokers may be positive for a couple of days. Less sensitive are saliva tests, which register positive for about 2-4 hours after smoking. On the other hand, if you have smoked recently, you may do better to take a urine test, since this will at least leave open the question as to whether you were under the influence. Also, urine doesn't turn positive until several minutes after smoking.