What is the shelf life for urine. If a test was for drugs, and nothing was found. Can that same sample be retested 3 weeks later to determine a b.a.c at the time of collection?

Probably not but that will depend on the specific laboratory and several other factors. UA samples are usually taken and shipped at room temperature and without preservative. Unpreserved (for alcohol) and at room temperature the test range is 2 days, refrigerated- 2 weeks, frozen- 4 weeks.

If being used in any type of court action, a chain of custody form will disclose who handled and storage information.