An Introduction to Testing for Drugs of Abuse. William E. Schreiber

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      Urine drug testing is ordered for a number of reasons.

       Confirm or rule out a suspected acute drug intoxication (eg, patient in the emergency room with symptoms of a drug ingestion).

       Discover use of illegal or prohibited drugs (eg, employment drug screening program).

       Monitor patients with a history of drug abuse (eg, drug rehabilitation program).

       Ensure compliance with drug prescriptions that might be diverted (eg, opioid screening in pain management clinics).

      Urine is easy to collect and does not require special handling. Point‐of‐care screening tests can be performed directly on the sample with no additional preparation. Most compounds of interest are excreted in urine, so recent consumption of a drug is likely to be detected in this sample.

      The main drawback of a urine sample is that it can be tampered with by the owner before analysis. The issue of specimen validity testing (SVT) is discussed later in this chapter.

      Oral Fluid

      Oral fluid consists mainly of the mixed saliva that comes from three major and several minor salivary glands. Small amounts of cellular debris, bacteria, and the residue of ingested substances are also present. It is more than 99% water and contains electrolytes, proteins, enzymes, and other biomolecules.

      Drugs appear in oral fluid by passive diffusion from blood. The rate of transfer is related to the chemical properties of a drug and the degree of binding to plasma proteins (only free drug molecules can diffuse across capillary walls).

      The window of detection is several hours to several days following drug use, depending on the specific drug and the amount and frequency of consumption. One advantage of oral fluid over other samples is that recently ingested (e.g., ethanol) or smoked (e.g., tetrahydrocannabinol) drugs may leave a residue that is present in high concentration.

      Oral fluid is a noninvasive specimen and is easy to obtain with commercial collection devices. The collection process can be witnessed to prevent sample adulteration or substitution. It is especially useful in roadside testing for drugs that impair a person's ability to drive.

      Hair

      Drugs in blood diffuse into the hair follicle, where the hair shaft grows. As the keratin matrix of hair is formed, drugs are incorporated and remain there, moving outward as the length of the hair shaft increases. A human hair is therefore a biological record of drug exposure.

      The unique advantage of hair testing is the extended time period over which drugs can be detected. Hair samples can test positive as early as 1 week following drug use, and the window of detection is months or even years, depending on hair length.

      Hair is usually collected from the crown of the head. The average growth rate for hair at this site is about 0.5 in (1.3 cm) per month. Sample collection is fairly simple – multiple strands of hair are cut at scalp level and sent to a testing facility for analysis. Collection can be witnessed or performed by trained personnel to prevent substitution or adulteration of the hair sample.

      The laboratory may test a defined length of hair, which corresponds to an approximate period of time (i.e., hair trimmed to 1.5 in will detect drugs taken within the past 3 months). It is also possible to cut hair into short segments, which are analyzed separately. By comparing the test results on adjoining hair segments, an approximate timeline of drug exposure can be determined.

      Analysis of drugs in hair is technically more demanding than other specimens. Hair samples must be washed to eliminate any surface contamination, and drugs must then be extracted from the hair shaft into a liquid medium that can be analyzed. This is a specialized procedure that is not available in most routine toxicology laboratories.

      Other Specimens

      Other sample types are informative in specific situations.

       Meconium is the greenish‐black stool passed by a newborn following delivery. Analysis of this material can detect drug exposure in utero (i.e., drug use by the mother during pregnancy).

       Sweat can be collected with an absorbent skin patch for up to 1 week. Analysis of the accumulated sweat will indicate whether drugs have been consumed during that time period.

Specimen Validity Testing

      Many people are required to undergo drug testing as a condition of employment or to remain in drug rehabilitation programs. Testing is most often performed on urine because it is easy to obtain, and both drugs and their metabolites are present in concentrated form.

      How Cheaters Try to Beat Drug Tests

      Collection of urine samples is not usually witnessed. The person providing the sample may try to get a “clean” test result by tampering with the specimen before submitting it. Specimen tampering can be divided into three categories.

       Adulteration – a substance is added to the specimen that masks the presence of drugs and their metabolites. The substance may interfere with the analytical method or it may react with the drug(s) of interest to produce compounds that are not detected. Bleach, vinegar, ammonia, and other household products have been used for this purpose. More sophisticated operators use additives that interfere with drug tests but are hard to detect. These adulterants include nitrite, chromate, peroxide, halogens (all of which are oxidizing agents), and glutaraldehyde. Commercial products with amusing names like Whizzies, UrineLuck, and Stealth are available on the internet and in drug paraphernalia shops.

       Substitution – the patient substitutes another sample for their own urine. It could be a friend's urine, a synthetic urine product, a yellow‐colored fluid such as apple juice, or tap water. Since the sample does not come from the patient, it will test negative (unless they pick the wrong person to provide the substitute sample!).

       Dilution – the goal of this strategy is to dilute drugs in urine below the threshold for a positive result. This can be accomplished in several ways: (i) drinking an excess of fluids prior to collection, (ii) taking diuretics to flush drugs out of the urinary tract, and (iii) adding water or another liquid to the sample itself.

      Assessing Validity

      Assessment of specimen validity begins with measuring urine temperature, which should be 90–100 °F (32–38 °C) within 4 minutes of collection. The physical appearance is also evaluated for color and consistency, and any unusual odor is noted.

      The following chemical tests are run on patient samples to detect tampering that may give a false result.

Creatinine	Creatinine is excreted in urine and serves as a marker for this fluid. Random urine specimens contain at least 20 mg/dL (1.8 mmol/L) of creatinine. Values below this level may indicate a dilute or substituted sample
Specific gravity	This test measures the number of solute particles in urine. The reference range for specific gravity СКАЧАТЬ В начало < 7 8 9 10 11 12 13 14 > В конец