The problem is reagents combine with substances similar to drug metabolites. Hence Advil, Sinex or other medicines may be similar enough to certain illegal drugs to cause a positive reaction. This test is somewhat more sophisticated and more expensive than the EMIT test. Produced by Roche Diagnostics Inc. under the name Abuscreen, this test is occasionally used by the armed forces. This more complicated procedure involves adding a radioactive antigen to the sample of urine and analyzing it by a machine.
Mistakes come from poor calibration.
This stands for thin layer chromatography. The procedure involves adding solvent to urine to extract drugs and then comparing color spots on a TLC plate to that of a standard. TLC relies on the subjective judgment of a technician and requires considerable skill and training. False positives result from misinterpretations. It is not widely used.