Dissimilar pharmacokinetics make blood and urine testing useful for different purposes in compliance monitoring.
During his presentation, "Comparative Interpretation of Urine and Blood Toxicology for Compliance Monitoring," toxicologist Joshua Gunn, PhD, discussed the utility of blood and urine testing as a compliance-monitoring tool. Noting "the very different pharmacokinetics of blood and urine," Gunn said that blood and urine testing cannot be compared directly as they provide different information. Urine "is a more suitable matrix for identifying illicit or non-prescribed drug use," whereas blood is more suitable for evaluating the patient's compliance with a prescription drug regimen.
Gunn said that certain characteristics of urine testing make it irreplaceable for compliance monitoring, noting that drugs are retained for longer periods of time in urine than in blood, making urine testing an ideal tool for providing a history of a patient's recent use. Other benefits include the non-invasive nature of urine collection and ease of obtaining adequate sample volumes for testing.
Although urine testing is a useful tool for monitoring illicit drug use, according to Gunn it has "certain limitations when it comes to the field of compliance monitoring." There are many variables involved in the urine metabolism/excretion process, including individual variability in renal capacity and pharmacogenetic makeup; enzyme inhibition, activation, and competitive metabolism related to concomitant drug use; urine pH; and the patient's level of hydration. Although urine testing can tell a clinician if a patient has recently taken prescribed medication, it cannot provide any information on the dosing habits of the patient, because there is no relationship between urinary drug concentrations and the ingested dose, Gunn said. "The presence of a drug or metabolite in urine simply identifies recent use," he said. "This is not a concern for all patients but in cases where quantitative information pertaining to the prescription drug is desired, blood provides a more suitable matrix."
Compared to urine testing, blood testing provides "pivotal information pertaining to dosing habits," Gunn said, and can be used to better understand the disposition of any drug in the patient. To further illustrate the differences between urine and blood testing, Gunn asked the audience to consider the examples of a patient who is taking one pill every other day and diverting the rest of his prescription, and a patient who is taking the prescribed dose of his prescribed medication but is also obtaining additional drugs from other providers. Gunn said that urine toxicology would not be able to provide this information or enable the clinician to confirm if these patients were taking their meds as prescribed.
Gunn said that clinicians should think of blood testing results as a "snapshot" of what was in the patient's system at the time the specimen was taken, whereas urine testing "provides a picture of what drugs the patient has been taking for the last three or four days." Combination blood and urine testing can give a fuller clinical picture of a patient's drug-taking habits and history. Gunn said that the "disposition and retention times of drugs in blood verses urine are quite different," and this difference "provides the basis for differing blood/urine toxicology reports." When collected together, "blood and urine provide the most information possible and aid in identifying pill scraping, patients taking infrequent doses, patients dosing shortly before their office visit, and patients who are over-medicating."
Gunn concluded by reminding the audience that all patients should undergo urine drug testing to minimize the risk of drug abuse and diversion. He also recommended that, when treating long-term, tolerant opiate users, providers should consider drawing patients' blood once a year to establish steady-state blood levels. "Urine drug testing is a tool that should be used not only to identify non-compliant patients but also compliant patients," he said.