Results of a urine test show the presence or absence of specific drugs or drug metabolites in the urine. Metabolites are drug residues that remain in the system for some time after the effects of the drug have worn off. A positive urine test does not necessarily mean the subject was under the influence of drugs at the time of the test. Rather, it detects and measures use of a particular drug within the historical "look-back" time of each particular testing drug ingested. Almost all drugs will show up in urine. Some of the most common attempts at adulterating (trying to invalidate the specimen) are adding water, bleach, soap, etc to the sample. Some will even go to the internet to see if they can purchase something they can take to ruin the sample.
The results are reliable although they donor can try to adulterate the sample. This is also less expensive than hair testing and will detect all illegal drugs.
Adulteration is possible. There are cases where the donor has to be monitored or actually observed. As an example, some probation cases require an observed collection. This is the case if the donor has tried to adulterate a previous sample. It is considered a possibly embarrassing method of collection.
Times varies with respect to the drug you're looking for, the persons metabolism, the amount used, the weight of the person tested. As an example, you can detect marijuana up to 45 days if the person is a heavy user.