From US Drug Test Centers:
How to Avoid a Dilute Specimen
- It is always best to go for a urine collection first thing in the morning because your urine is fresh and not likely dilute. Do not drink extra water because you are afraid of not being able to produce urine at your drug test collection. If you normally drink large quantities of fluids, cut back a bit before going for your drug test.
- Drinking an excessive quantity of fluid before a drug test can cause dilution and then you might have to back for another drug test.
- If, at the collection site for your drug test, you have any difficulty providing the required 45 mL of urine, you will be given time to drink some water and try again. We call this a “shy bladder.” There is a shy bladder process where you have up to three hours to provide your specimen and you are given a maximum of 45 ounces of water spread out over the three hours.