An untampered batch of synthetic urine contains no drug metabolites to test positive for. Therefore, any anecdotes about synthetics testing positive for drug metabolites should be taken with a box of salt.

Labs have been checking for non-human samples for several years. Under DHHS regulations, a "non-human" sample is ruled when the creatinine concentration is less than 2 mg/dL and the specific gravity is less than or equal to 1.0010 or greater than or equal to 1.0200 on both the initial and confirmatory creatinine tests (i.e., the same colorimetric test may be used to test both aliquots) and on both the initial and confirmatory specific gravity tests (i.e., a refractometer is used to test both aliquots) on two separate aliquots.

As long as the synthetic batch is unexpired and had been stored properly (ie no exposure to direct sunlight), it will pass the validity checks permitted by DHHS.

If youre concerned, you can always test your QF batch with a validity test strip to see if the creatinine, SG etc, are all within acceptable ranges. These tests are inexpensive and can be found online by doing a Google search for "drug adulteration test kits".