Urine drug testing can be effective in controlling drug misuse, but it has its limitations. It is important never to react to a result until all of the possible explanations have been considered.
Reasons for drug testing:
(1) to confirm compliance
(2) to detect possible diversion
(3) to detect substance misuse
Reasons for a negative urine drug test due to patient misbehavior:
(1) urine dilution (may be detected by low urine creatinine and specific gravity)
(2) urine adulteration (testing for adulterants can detect)
(3) urine substitution (minimized by scrupulous adherence to urine collection procedures)
(4) drug diversion
(5) noncompliance
Reasons for a negative drug test due to testing error:
(1) deterioration in the test system (a problem for urine cups with built in detection systems that lack internal control) or other laboratory error
(2) urine test does not detect the drug (for example, a urine test for opiates may detect only derivatives of opium and not synthetic opioids)
Causes for a negative drug test in a compliant patient:
(1) level of drug below the cutoff (testing to the analytical level of detection can expand the range of detection)
(2) intended drug dosage results in very low blood levels, far below cutoff (an example is transdermal fentanyl)
(3) rapid drug clearance (rapid metabolism, drug interaction, etc)
