A body fluid frequently used in forensic toxicology, occasionally as physical evidence, and as a source of cellular material for DNA typing. Urine contains urea, creatinine, uric acid, and a number of ionic materials such as chloride (C1-) and phosphate (PO43″). If a stain is encountered that might be urine, the simplest presumptive test is based on smell. Upon heating, urine gives off a characteristic and distinctive smell. There are also gel-based diffusion techniques that can be used. With the advent of DNA typing, it is sometimes possible to type the cellular material excreted in urine. However, the sample must be fairly concentrated for this to be an option.
By far the most important role of urine in forensic science is in the area of toxicology, in which it is a preferred medium for detection of many drugs and poisons. Traces of drugs and metabolites remain in the urine much longer than in blood, and it is generally easier to quickly screen a urine sample than it is to screen a blood sample. Urine is the principal medium for workplace and employment drug screening, which is often referred to as urinalysis. Analysis of urine for drugs proceeds as does any other drug analysis, from presumptive and screening tests such as thin layer chromatography (tLc) and immunoassay to specific confirmatory analysis such as gas chro-matography/mass spectrometry (gc/ms). In the case of employee drug testing or drug testing for athletes, the toxicologist must also check for purposeful adulteration of the sample to mask the use of drugs. Such adulteration can range from simple dilution to the addition of masking compounds either directly to the urine sample or indirectly by ingestion. Although urinalysis is effective and efficient, the levels of a drug or metabolite in urine cannot be directly related to a level of impairment. In addition, urine reflects only relatively recent ingestion of substances (within a week or so depending on the drug) and will not reveal drug use that occurred before then.