Synthetic urine should not be used if a full-blown medical test is performed. The synthetic urine would conceal any diseases or other health-related issues that you may not be aware of in which the medical test would bring to your attention.

Is it possible to mix synthetic urine with semi-clean urine so metabolite levels are low while other characteristics of the urine (creatinine, hormones, odor, color, etc.) remain natural?
For the sake of argument - If the 'semi-clean' urine sample contains metabolite levels low enough to register below the cutoff limit of the drug screen, then why not use it as a whole instead of mixing it with synthetic?
And how would you know how low the levels were without paying to have a quantitative analysis (GC/MS) performed? The Immunoassay screen (as well as the home assays) wouldnt tell you diddly-squat. They are a pass/fail endeavor.