2 ounces or 3?

Ok, what is the story with the volume on these screens? Is there any documentation that specifies the minimum amount that is required for these urine screens? I thought I read somewhere that 2 was minimum but then there are people on here that say they have been asked for more ( I wouldn't give them more myself). I worked as a medical technologist in a lab running drug screens from 1997 - 2001 and the amount you use for the assay is a few milliliters. So 2 ounces is way more than really necessary. The collection cups that I have seen show that 50 mls or 2 ounces is the top line on the cup.