Also, the way I understand it, a metabolite is nothing more than a protein structure. If that protein becomes degraded (by excess heat/light exposure, exogenous enzymatic degradation, etc) it is rendered undetectable since that protein no longer appears the same way on chromatography. Again, I am not a doctor, but only say what I know from 15 years of attempting to circumvent testing. No doubt, the sensitivity and specificity of testing modules, as well as the protocol and modules themselves likely have changed drastically.