There is no particular reason for using urine as a specimen. However, it is preferred over other specimens such as blood, hair, saliva, etc. because it is relatively easier to collect urine as a drug testing sample. Urine is also considered as the best specimen to use because firstly, it causes little hindrance in drug testing assays unlike other samples such as blood. This is relatively because it contains lesser lipids and proteins as compared to blood because of which lesser interferences occur while testing. Secondly, the drug concentrations are comparatively higher in urine rather than blood or other specimen. Therefore, it is preferably more convenient to use urine as a testing sample.
SAMSHA (via DWP) has considered other alternate samples to use for drug testing, urine deemed to the most feasible one amongst them. However, urine is not reliable to test the frequency of drug usage as many drugs are immediately cleared from the body and if, single dosage of drugs is taken weeks prior testing, it would not be detected.
Therefore, generally 3 alternate samples are taken. Many laboratories also use sweat patches to detect long term drug usage as the drugs are distributed in every part of the body and appear in sweat as well. But, the sweat patch is not commonly used. It is only used in cases where long term monitoring is done such as that of prisoners, etc.
Many laboratories advocate the use of hair as a specimen to detected drug for long term period. This is so because when the drug spreads to every part of the body, it is also incorporated into the hair as it is formed and then grows out. So, hair can be used for knowing long term drug history. However, a restriction with hair sample is that it doesn’t indicate any relative changes which occur in the drug frequency levels of the body. Hair show lesser drug concentrations as compared to urine samples so they are not too reliable to exactly measure the quantity of drug level. Also, hair are exposed to be contaminated with outside impurities so prior to sample taking, hair are washed. But it is presumed, that the drugs in the external contamination penetrate the hair follicle, because of which ambiguity occurs in the results.
When considering specimens, oral fluid is also used. There is no sure relation between the concentration of drugs in oral fluids and the mutilation occurring due to it, but it is used to indicate that the drug is circulating in the blood stream. It also used to indicate how recently the drug was used. Being the least insidious technique to collect specimen, it used commonly used to screen drug impaired drivers at road testing and also for workplace drug testing.