When an employer requests a drug test from an employee the employee is typically instructed to go to a collection site. The employee’s urine is collected at a remote location in a specially designed secure cup, sealed with a tamper resistant tape, and sent via express delivery service to a testing laboratory to be screened for drugs (typically SAMHSA 5 panel).
The first step at the testing site is to split the urine into two aliquots. One aliquot is first screened for drugs using an analyzer that performs immunoassay as the initial screen. If the urine screen is positive then another aliquot of the sample is used to confirm the findings by
About 5% of the all pre-employment urine samples tested in the U.S. turn out positive for drugs. The percentages are much higher for post-accident and/or reasonable suspicion testing.
The efficacy of urine testing is debatable due to systematic cheating (adulteration and/or substitution of the specimen). Sample substitution and adulteration occur frequently, and both are effective methods of avoiding would-be positive tests. While most laboratories performing drug screens test for adulterants routinely, they can not keep pace with the various forms of adulterants available. 
Adulterants can be classified into two categories. The first category includes classic adulterants comprising household substances and other chemicals such as chlorine bleach, liquid drain cleaner, white vinegar, glutaraldehyde, sodium nitrite, and pyridinium chlorochromate. The second category includes commercial adulterants, which can be purchased from certain specialty stores (so-called “head shops”), through magazine advertisements, or via the Web. This second class of adulterants includes brand names such as Urine Luck version 6.3, Stealth, Clean-X, Klear, Purafyzit, Instant Clean, Krystal Kleen, and UR’n Kleen. Some of the commercial adulterants represent a repackaging of classic adulterants. One adulterant, for example, Klear, is made of nitrite and affects the tetrahydrocannibinol (THC) confirmation process but not the immunoassay process. Urine Luck 6.3 is actually hydrofluoric acid, which is an oxidant not generally available in the classic adulterants category because of its corrosive nature. Stealth is an enzyme system that generates hydrogen peroxide.
Abusers can also drink 1 to 2 liters of water to successfully dilute the concentration of drug metabolites in their urine below detectable thresholds, without risking detection. While this results in clear samples that may be rejected on the grounds of being too dilute, the practice is typically successful. A B vitamin will make urine yellow despite this practice of waterloading.
Adulteration tests can detect the use of some of these “detox” products. Adulteration tests analyze the properties of the urine to identify if the sample is abnormal. Such properties include the pH and specific gravity of the urine. They also test the level of creatinine, nitrite, glutaraldehyde, specific gravity, bleach and pyridinium chlorochromate.
Also, the wide availability of at-home drug screens allows an individual to take their own test before they receive one, thus knowing the results ahead of time- giving the user further opportunity to dilute the sample or to find a substitute.
New regulations from the