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Breast milk can be frozen and used later.
If someone is clean at the time and choose to freeze their urine at -18°C, and thaw it many months later and mix thoroughly, would it constitute back to something comparable to fresh urine in lab's point of view?
Only consideration would be temperature if the lab is expecting the urine to be given by the subject at the time it is taken into custody. Otherwise yes, it should be fine. Drug testing facilities freeze urine for sample storage all the time.
Agreed with the temperature issue you'd have to ensure it was warmed up to the approprate temperature. I would buy a home testing kit or something on definitely positive urine to make sure that it showed up (if that doesn't show and you know its positive you could assume that everything else in their including the controls they test for will also not show).
Whislt you can freeze it you'd want to make sure that if the temperature it is going to be stored at is -18C that it remains at that temperature, putting it in the door of a freezer will result in areas of it going to temperatures where they begin to melt.
The constant cycle of freeze thawing will most likely be something they'd pick up on as the levels of the controls they use to make sure its urine will be damaged. In laboratories using biological samples (blood, urine, cells etc.) -20C is the temperature they are stored at for less than a year. Precious samples (as your urine would be) are placed at the very back in the coldest region possible so the temperature does not really deviate at all because of this, when they recieve something that will be used multiple times they will make lots of containers of small volumes as well.
So summing up if you freeze and then thaw it once it might actually work however if it is not stored approprately (and you'd want to be certain about it being stored properly) then they'll see something is wrong and you'll be screwed. The use of old urine looks pretty bad as they'd see it as a person who was planning on continuously performing illigal activities who even planned ahead to make sure they could continue doing this.