I’ve only found the generic answer of yes diabetes/kidney infection/kidney disease/renal failure can cause a false positive for benzoylecgonine on a drug panel screening but not how. What is the mechanism that causes this? 

If you're taking Metformin for Type II, there's a chance it will flag for methamphetamines, or so a friend claimed.

47 minutes ago, Phi for All said:

If you're taking Metformin for Type II, there's a chance it will flag for methamphetamines, or so a friend claimed.

That sounds right. I am not doing urine analyses anymore but I do recall that this was something that was an issue back when we were developing assays. Not sure how common the effect is, though as I think it was more or less based on a single report.

22 minutes ago, CharonY said:

That sounds right. I am not doing urine analyses anymore but I do recall that this was something that was an issue back when we were developing assays. Not sure how common the effect is, though as I think it was more or less based on a single report.

Does a re-test reveal the false positive? I'm assuming this common oral medication doesn't set off flags every time with every person (I'm also assuming re-testing is allowed on a drug screening that detects cocaine or methamphetamine). 

35 minutes ago, Phi for All said:

Does a re-test reveal the false positive? I'm assuming this common oral medication doesn't set off flags every time with every person (I'm also assuming re-testing is allowed on a drug screening that detects cocaine or methamphetamine). 

OK, I found my old notebook. So the assay for amphetamine testing is (I think) an ELISA test which turns positive for unknown reasons. Based on that I would assume that doing it again would create similar results, but the report did not show quantitative analyses how reproducible it was (i.e. out of 10 tests how often would it show positive vs negative?).

If we do a chemical screen (using mass-spec) we can actually search for masses corresponding to amphetamines and that is what was done in the case study and that screen came up clean.

Perhaps a more general answer which OP might be looking into: I do not know the precise mechanism for FP in benzoylecgonine or other compounds. The main issue is that most of the time a immunochemical assay is used, which relies on antibody-antigen binding (or more typically, competition of binding). Now false positives are typically down to something that competes with your reagent standard but which is something you do not want to test for.