Individual patients may have genetic variants of the enzymes, or may be taking inducer or inhibitor drugs, that strongly influence opioid metabolism and, hence, their detectable presence or absence
in urine (or blood, or oral fluid if used) [see, Carlozzi et al. 2008; Smith 2009]. For example, the unexpected absence of one of the above opioid analgesics upon testing can be due to rapid metabolism in a patient rather than therapeutic noncompliance or drug diversion.