Just a little warning to you and any others reading this thread - I've had two urine tests for drug use over the last 3 years. My first - not only did I test positive for opiates, they also told me that they knew I tried to dilute my urine (I drank lots of fluids prior to the test) because of the low creatinine levels. They told me that I didn't have had to repeat the tests because they proved I had opiates in my system anyway. I found an interesting article that I highly recommended you read and pass on to everybody you know who my find themselves in this situation.
http://www.dalpa.com/public/drug/independent/urine.htm