3.1. Urine Specimens
By far, urine is the most widely used specimen for drugs-of-abuse testing. It offers the advantages of large specimen volume and relatively high drug concentrations, because of the approx 100-fold concentrating effect of the kidneys (each minute, approx 125 mL of blood plasma is filtered in the kidneys by the glomeruli and concentrated to approx 1 mL of urine). There is an extensive body of literature addressing the detection of drugs and their metabolites in urine specimens, and much is known about the pharmacokinetics of drug and metabolite elimination in urine. There are several well-established guidelines and laboratory certification programs, most notably those originally established by the National Institute on Drug Abuse (NIDA) in 1988 for federally regulated workplace drug-testing programs. These guidelines, called the NIDA Guidelines, are now overseen by SAMHSA (8). These guidelines address testing for five drugs of abuse: cannabinoids (marijuana), cocaine metabolites, opiates, amphetamines, and phencyclidine (PCP). These workplace drug-testing guidelines are widely regarded as the "gold standard" in urine drug testing. As of March 2005, there were 49 laboratories (including 2 in Canada) certified under this program to perform such federally regulated workplace drug testing.
Urine is 95% water, with sodium chloride and urea in about equal amounts as the main dissolved substances, and with much smaller amounts of a wide variety of other constituents. Urine is attractive as a specimen because it can be conveniently provided as a normal waste product in relatively large volumes. Typical urine production rates are about 1 mL/min during waking hours, so collection of a specimen of sufficient volume for both initial screening by immunoassay as well as any subsequent confirmation testing is generally not problematic. Furthermore, the large specimen volume allows the option of splitting the specimen into two portions at the time of collection for assurance of proper chain of custody and specimen integrity in the event of adversarial challenges to the test results. Drugs and their metabolites are reasonably stable in urine when specimens are refrigerated or frozen. As far as specimen handling is concerned, urine is generally not considered infectious unless visibly contaminated with blood (5).
Urine is relatively easy to collect and analyze. There are a wide variety of immunoassays available for detection of most common drugs of abuse and/or their metabolites in urine. Furthermore, in part because of the relatively high drug and/or metabolite concentrations in urine, simple noninstrumented, on-site immunoassays have been developed and are widely used in a variety of settings. There are numerous versions of these simple-to-use, noninstrumented immunoassays (i.e., visually read dipsticks, cassettes, cups) which allow the ready on-site testing of urine specimens outside of a formal laboratory. Some of these devices have even been cleared by the US Food and Drug Administration (FDA) for at-home use. Studies of the performance of these noninstrumented, on-site devices have demonstrated impressive performance for some devices at levels comparable to bench-top laboratory analyzers, even when performed by those without any formal laboratory experience (9,10).
In spite of the well-established place of urine as a specimen for drug testing, its use is not without its challenges. One issue is the potential invasion of privacy involved in specimen collection. Unlike within the criminal justice testing programs (pretrial, probation, prisons, drug courts, and so on) (see Chapter 15), most workplace and other drug-testing programs do not allow direct observation of specimen collection, except under special circumstances. Without direct observation, the opportunity for specimen adulteration and substitution exists (see Chapters 13 and 14). To respond to such efforts to thwart the integrity of the testing, specimen collection and laboratory guidelines have been developed to minimize the opportunities for such tampering. Furthermore, laboratory procedures to detect such efforts have been established. Adulteration is generally easily detected and is difficult at best when collection is performed under direct observation, as in criminal justice contexts. Although there are many adulterants available, there are also many simple test strips as well as laboratory methods to detect such adulterants.
Specimen dilution (in vivo), however, is a much greater challenge, as it is fairly easy to drink sufficient excess fluids prior to specimen donation and dilute one's urine by a factor of up to 10 or even more, thereby minimizing the chance of testing positive at conventional screening cut-offs. Although there are established criteria for what constitutes an excessively dilute specimen (e.g., creatinine less than 20 mg/dL and specific gravity less than 1.003), these regulatory criteria are undergoing scrutiny to ensure that false accusations of intentional efforts at dilution are not made.
Another limitation to the utility of urine specimens in drug testing is the relative difficulty in correlating urine drug and/or metabolite levels with likely dosing and likelihood of impairment. Granted, the correlation between urine drug levels and time and extent of drug use and likelihood of impairment is weak. Unfortunately, some toxicologists claim that urine drug levels should never be interpreted, but this clearly is an extreme and incorrect position. In some situations, urine levels may clearly be commensurate with the claims of the user or not, and as such can be highly useful. Furthermore, very high urine levels can clearly demonstrate recent and significant use, whereas low drug levels are much more difficult to interpret. But to deny any value in urine drug levels for interpretation is incorrect.
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