What type of job is this for? Too many do not understand the workings of an employment drug test, it is not a simple "presence/absence" test.
First, the sensitivity of the screen is dependant on the liability of the job, i.e. janitor vs. bus driver, bag boy vs. pilot. The former job type is not screened with a low cut off level (allows for detection of drug use over greater time frame), while the latter is, since the employer wants to determine if the person has used recently and also in the past for the jobs with great liability.
Second, there are panels of drugs that are screened for, and the number of drugs on the panel is determined by the employer, for he/she pays increasingly as they choose more and more drugs of abuse to detect. Also, the sensitivity of the screen increases the costs. So, you can actually be positive, even "on" a drug, when screened and pass the test, if that drug was not being screened for. Say, you were a heroin user...hell, you had a needle hanging from your arm when you pissed in the cup; you could "pass" this test if the employer only paid to screen for THC, cocaine, and benzodiazepines.
The reason why people have concocted all of these "methods" to evade drug tests is because they don't understand the test. Alot of the shit people take or do does absolutely nothing, they passed not because of the herbs or water, but because of the variability of the tests. They merely see the simple correlation and determine it to be causative.
Finally, a neat little bit of information is that the initial spectrophotometric screen that many labs do is sensitive to glucose. It will make the instrument record "0", which is not normal, since there is background "noise" at all times. They will screen the sample with a Chem-Strip, checks for pH, specific gravity, ketones, glucose, etc. (this is how they determine if the sample is true urine, as opposed to water...the specific gravity is different), and if it shows positive for glucose, they will consider the sample "negative" and assume diabetes.