The basic methods of testing for the presence of drug metabolites in a urine sample are thin layer chromatography, immunological assay chromatography, and gas chromatography/mass spectrometry, with the latter being the most accurate and reliable testing method. Regardless of the analytical methodology used, the evidentiary value of the laboratory results depends on proper sample handling techniques in conjunction with a full chain of custody. Most successful urinalysis legal challenges have focused on sample collection procedures and the protocol used in initially labeling bottles and sending them off to the laboratory. The most common defense to a positive test is the claim of the accidental ingestion of the identified substance. The passive inhalation of marijuana is the most common but least successful defense. Although accurate urinalysis data demonstrate the use of a drug, they do not indicate when the drug was taken nor the level of impairment. Clinical observation, surrounding evidence, and other tests are required for maximum results in a drug detection and prevention program.