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update:
For future readers it appears samples do degrade but depending on how the lab runs the test will decide if it comes up positive. Below is the SAMHSA (federal) guideline rules about retesting a sample where it addresses a sample degrading in storage.
(j) Retesting a Specimen for Drugs.
(1) A second laboratory shall use its confirmatory drug test when retesting an aliquot of a single specimen or testing a split (Bottle B) specimen for the drug or drug metabolite that was reported positive in the single specimen or the primary (Bottle A) specimen by the first laboratory.
(2) Because some drugs or drug metabolites may deteriorate during storage, the retest of an aliquot of a single specimen or the test of a split (Bottle B) specimen is not subject to a specific drug cutoff requirement, but must provide data sufficient to confirm the presence of the drug or metabolite.
(3) If the second laboratory fails to reconfirm the presence of the drug or drug metabolite that was reported by the first laboratory, the second laboratory shall attempt to determine the reason for not reconfirming the presence of the drug or drug metabolite by conducting specimen validity tests. The second laboratory shall conduct the same
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specimen validity tests it would conduct on a single specimen or a primary (Bottle A) specimen. The second laboratory reports all test results to the MRO.
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