Properly performed urine drug testing involves two steps: an initial screening test followed by confirmatory testing for substances with positive screening results. Testing may be performed for reasons other than monitoring opioid therapy, 8 such as drug rehabilitation, employment requirements (e.g., for occupations that require special transportation licensing), military or sports participation, or legal situations. Urine drug testing can be performed in the office as a point-of-care test, or the sample can be sent to a reference laboratory for testing. 7 Urine drug concentrations do not reflect serum concentrations rather, they are a function of how rapidly a person metabolizes and eliminates the drug and its metabolites, as well as hydration status. However, urine testing is most common because of ease of collection, adequate sensitivity and specificity to detect commonly used drugs, and a longer window of detection than serum. Urine, serum, saliva, sweat, and hair can be tested for the presence of drugs. Family physicians must be familiar with these substances, the methods to detect them, and their effects on urine drug testing. Urine samples can be adulterated with other substances to mask positive results on urine drug testing. False-negative results can cause missed opportunities to detect misuse. Immunoassays can cross-react with other medications and give false-positive results, which have important implications for a patient's pain treatment plan. Immunoassays for opioids may not detect commonly prescribed semisynthetic and synthetic opioids such as methadone and fentanyl similarly, immunoassays for benzodiazepines may not detect alprazolam or clonazepam. They can give false-positive and false-negative results, so all results are considered presumptive until confirmatory testing is performed. Immunoassays are qualitative tests used for initial screening of urine samples. Family physicians should be familiar with the characteristics and capabilities of screening and confirmatory drug tests. Urine drug testing is an essential component of monitoring patients who are receiving long-term opioid therapy, and it has been suggested for patients receiving long-term benzodiazepine or stimulant therapy.
False-positive results have significant implications for a patient's pain treatment plan, and false-negative results can be a missed opportunity to detect misuse. All results are considered presumptive until confirmatory testing is performed. Immunoassays are used for initial screening and can give false-positive and false-negative results.
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