Evidence-Based Reviews

Urine drug tests: How to make the most of them

Author and Disclosure Information



Factors that can affect UDT results

In addition to knowing when to order UDT, it is critical to know how to interpret the results of UDS and follow up with confirmatory tests when needed. Other than the limitations of the tests, the following factors could contribute to unexpected UDT results:

  • the drug itself, including its half-life, metabolic pathways, and potential interactions with other medications
  • how patients take their medications, including dose, frequency, and pattern of drug use
  • all the medications that patients are taking, including prescription, over-the-counter, and herbal and supplemental preparations
  • when the last dose of a prescribed controlled substance was taken. Always ask when the patient’s last dose was taken before you consider ordering a UDT.

To help better understand UDT results, Figure 114 and Figure 215 demonstrate metabolic pathways of commonly used benzodiazepines and opioids, respectively. There are several comprehensive reviews on commonly seen false positives and negatives for each drug or each class of drugs in immunoassays.16-21 Confirmatory tests are usually very accurate. However, chiral analysis is needed to differentiate enantiomers, such as methamphetamine (active R-enantiomer) and selegiline, which is metabolized into L-methamphetamine (inactive S-enantiomer).22 In addition, detection of tetrahydrocannabivarin (THCV), an ingredient of the cannabis plant, via GC/MS can be used to distinguish between consumption of dronabinol and natural cannabis products.23 The Table16-21 summarizes the proto­type agents, other detectable agents in the same class, and false positives and negatives in immunoassays.

Metabolic pathways of commonly used benzodiazepines

Interpreting UDT results and management strategies

Our Algorithm outlines how to interpret UDT results, and management strategies to consider based on whether the results are as expected or unexpected, with a few key caveats as described below.

Metabolic pathways of commonly used opioids

Expected results

If there are no concerns based on the patient’s clinical presentation or collateral information, simply continue the current treatment. However, for patients taking medications that are undetectable by UDS (for example, regular use of clonazepam or oxycodone), consider ordering confirmatory tests at least once to ensure compliance, even when UDS results are negative.

Commonly seen false positives and false negatives in urine drug screens

Unexpected positive results, including the presence of illicit drugs and/or unprescribed licit drugs

Drug misuse, abuse, or dependence. The first step is to talk with the patient, who may acknowledge drug misuse, abuse, or dependence. Next, consider modifying the treatment plan; this may include more frequent monitoring and visits, limiting or discontinuing prescribed controlled substances, or referring the patient to inpatient or outpatient SUD treatment, as appropriate.

Continue to: Interference from medications or diet

Next Article: