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. In this, the most sophisticated test a sample of urine is injected into the machine. The urine separates as it travels from the injection port to the detector and as the sample emerges from the gas chromatograph and is ionized by electron bombardment. The resulting positive ion mass fragments are read by the mass spectrometer. The results are produced on a computer print out.