Diluted Urine

A negative result for the presence of abused drugs in a urine specimen does not mean that no drug is present. It is possible that the amount of drug is below the cut-off values for detection in the laboratory assays. Diluting urine is a simple way to make an otherwise positive drug tests negative if the original concentrations of drugs in the urine are slightly above the cut-off values. Federal guidelines recommend placing a toilet bluing agent in the toilet tank if possible so that the reservoir of water in the toilet bowl always remains blue. There should be no other source of water in the enclosure where urination takes place.

Consumption of a large amount of fluid prior to drug testing is a way to avoid a positive test (3). A creatinine concentration below 20 mg/dL or a specific gravity below 1.003 should be considered as an indication of diluted urine. Creatinine analysis in urine is a very effective method to detect diluted urine. Needleman and Porvaznik (4) considered a creatinine value of less than 10 mg/dL as suggestive of replacement of a urine specimen largely by water. Beck et al. (5) reported that 11% of all urine specimens submitted to their laboratory for drugs of abuse testing was diluted. The SAMHSA program does not currently allow analysis of dilute urine specimens at lower screening and confirmation cut-off values. However, in Canada, the Correctional Services of Canada (CSC), for diluted urine specimens, incorporates lower screening and confirmation cut-off for drug/metabolites (amphetamine: screening cut-off, 100ng/mL; confirmation cut-off, 100ng/mL; benzoylecgonine (BE): screening and confirmation, cut-off 15ng/mL; opiates: screening and confirmation cut-off, 120ng/mL; phency-clidine: screening and confirmation cut-off, 5 ng/mL and cannabinoids: screening cutoff, 20ng/mL; confirmation cut-off 3 ng/mL). Fraser and Zamecnik reported that 7912 urine specimens between 2000 and 2002 analyzed collected by the CSC were dilute, and out of that 26% screened positive using SAMHSA cut-off values. When lower values for cut-off and confirmation were adopted, 1100 specimens tested positive for one or more illicit drugs. The positive rate of diluted specimens was 18.2% in the CSC Institutes and 22.3% in parolee specimens. The drug most often confirmed positive in a diluted specimen was marijuana. Codeine and/or morphine were also commonly confirmed in these urine specimens and ranked second after marijuana in prevalence (6). Soldin (7) reported earlier that there was more than a 100% increase in cocaine-positive specimens when the cut-off was lowered to 80 ng/mL from 300 ng/mL in a pediatric population because neonates are not capable of concentrating urine to the same extent as adults. Luzzi et al. investigated the analytic performance criteria of three immunoassay systems [EMIT, Beckman EIA and Abbott fluorescence polarization assay (FPIA)] for detecting abused drugs below established cut-off values.

The authors concluded that drugs can be screened at concentrations much lower than that of established SAMHSA cut-off values. For example, the authors proposed a THC-COOH cut-off value of 35 ng/mL using EMIT and 14ng/mL for the Beckman EIA and the Abbott FPIA, where SAMHSA guidelines stated a cut-off value of 50 ng/mL. The proposed cut-off values were based on impression studies where coefficient of varience (CV) was less than 20%. Such lowering of cut-off values increased the number of positive specimens in the screening tests to 15.6%. A 7.8% increase was also observed in the confirmation stage of drugs of abuse testing (8).

New SAMHSA regulations (49 Code of Federal Regulation Part 40) indicate that a specific gravity lower than 1.0010 (new refractometers are capable of detecting such low concentration to four decimal places accurately) or above 1.020 and creatinine concentration less than 5 mg/dL are inconsistent with normal human urine. Edgell et al. performed a controlled hydration study with 56 volunteers to investigate whether it is possible to produce such diluted urine. Subjects were given 2370 mL of fluid, and urine specimens were collected at the end of each hour for a 6-h test period. No urine specimen satisfied the paired substitution criteria (specific gravity < 1.001 or above 1.020 and creatinine <5.0mg/dL) for diluted urine (although 55% subjects produce at least one dilute urine specimen during the first 3h of hydration with creatinine <20mg/dL and specific gravity <1.003). This supports the criteria set by SAMHSA for classifying a specimen as substituted (9). Barbanel et al. studied specific gravity and/or creatinine concentrations in 803,130 random urine specimens submitted to the laboratory. Out of these, 13,467 specimens had both creatinine and specific gravity measurements and none of them met the lower limit of specific gravity (1.001) and creatinine (5mg/dL). The patients who met one of the two criteria (creatinine <5.0mg/dL or specific gravity <1.001) were neonatal or so severely ill unlike anyone in the work force undergoing testing for abused drugs. Eleven patients met the criteria of substituted urine (creatinine <5mg/dL, specific gravity >1.020), but all of them were seriously or terminally ill (10). Cook et al. (11) demonstrated that an osmolality substation cut-off of <50m0sm/kg can be indicative of substituted urine.

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