Urine drug testing is commonly performed in both clinical and forensic arenas for screening, monitoring and compliance purposes. We sought to determine if urine creatinine concentrations in monitoring program participants were significantly different from hospital in-patients and out-patients undergoing urine drug testing. We retrospectively reviewed urine creatinine submitted in June through December 2015 for all specimens undergoing urine drug testing. The 20,479 creatinine results were categorized as hospitalized patients (H) and monitoring/compliance groups for pain management (P), legal (L) or recovery (R). Median creatinine concentrations (interquartile range, mg/dL) were significantly different (P < 0.001) between groups: H 126 (122-136); P 138 (137-143); L 147 (144-154); R 95 (92-97). In the two groups subject to on-demand sampling time pressures, median creatinine concentrations were significantly lower in the R vs. L group (P<0.001). In conclusion, recovery (R) participants have more dilute specimens, reflected by significantly lower creatinine concentration and may indicate participants' attempts to tamper with their drug test results through dilution means.