[ Home | Contents | Search | Next | Previous | Up ]
From: Robert Swotinsky MD
Date: 22 Oct 2005
Time: 17:09:02 -0700
Remote Name: 68.166.234.126
Yes, agreed: Human urine contains creatinine. But, under the regulated model of drug testing, a specimen is not deemed substituted unless both the specific gravity and creatinine meet substituted criteria. With the exception of saline specimens (where the specific gravity is in normal range), one would expect the specific gravity to be out of range if the creatinine is also out of range. The question posted here is, Why not accept an out of range creatinine alone as proof of substitution? There are at least two good reasons why the definition includes both specific gravity and creatinine: (1) Review of large datasets of paired specific gravity and creatinine values from human specimens (e.g., from hospital labs) identifies some specimens as just one of the values out of range, but no specimens having both values out of range. Thus, for whatever reason, some real urine specimens have registered out-of-range creatinine values. (2) The validity testing performed by labs has not in the past been subject to the same degree of quality assurance as the drug testing. Regulators are understandably cautious, and allow a generous margin for error, when mandating disciplinary consequences based on validity test results since they are not known to have equivalent reliability as has been established for drug testing.