As a laboratory procedure readily performed in physician office settings, the simplicity of urine drug screening belies its importance and impact, as test results can be used to monitor patients’ medication compliance,  detect  drug abuse,  provide evidence in legal/forensic cases, and improve  workplace safety.

Urine is the preferred specimen for drug testing primarily because it is non-invasive.. Urine specimens may contain detectable levels of drug over an extended period and at much higher concentrations than in blood. Urine may also contain higher levels of drug metabolites than blood, providing further evidence of drug use.

The immunoassay procedure,,  such as Elisa or RIA,  is performed first as a screening method.  If the immunoassay is negative, no further action is required, and the results are reported as negative.   If the sample is positive, the more specific GC/MS is used as a confirmatory test to identify individual drug substances or metabolites and quantify the amount of the substance.  Confirmatory tests, such as GC-MS should be utilized prior to reporting positive drug test results.

A summary of the many reasons that urine drug screens may be ordered:

  • Pre-employment
  • Suspicion of drug abuse (e.g., unexplained negligence/impairment/behavior)
  • Random testing outlined in employment contract
  • Military service
  • Sports participation
  • Legal/criminal (e.g., post-accident, parole, date-rape)
  • Drug-therapy compliance monitoring
  • Drug abuse rehabilitation monitoring
  • Postmortem investigation

Because of the personal, occupational, and legal implications that accompany drug testing, family physicians who perform urine drug screenings must be confident in their ability to interpret screening results and respond appropriately to that interpretation.

Ordering and interpreting urine drug screenings requires an understanding of the test procedure, the detection times for specific drugs, and the common reasons for false-positive and false-negative test results.

Possible reasons for false negative results:

False negatives are uncommon but can occur as a result of low drug concentrations in the urine, tampering, and in other situations. Possible reasons for false-negative results include:

  • Dilute urine (excess fluid intake, diuretic use, pediatric sample)
  • Infrequent drug use
  • Prolonged time since last use
  • Recent ingestion
  • Insufficient quantity ingested
  • Metabolic factors
  • Inappropriate test used
  • Elevated urine lactate
  • Tampering:

Possible Reasons for false positive results:

Although immunoassays are very sensitive to the presence of drugs and drug metabolites, specificity and accuracy varies depending on the assay used and the substance for detection. This limitation may result in false-positives from substances cross-reacting with the immunoassay.  Many prescription and nonprescription substances have been reported to cross-react with immunoassays and cause false-positives. Most have only been documented in case reports.. The frequency of false-positives varies, depending on the specificity of immunoassay used and the substance under detection.

In short, the importance and impact of urine drug screen results on the life of the individual tested requires a heightened awareness of both the strengths and limitations of the methodologies used.


[i] Urine Drug Testing: Approaches to Screening and Confirmation Testing.  Gifford Lum, MD and Barry Mushlin, MA. Laboratory Medicine.  June 2004. No.6 V. 35.
[ii] Drug Information Group.  University of Illinois, Chicago College of Pharmacy.  Feb. 2011.
[iii] Drug Information Group.  University of Illinois, Chicago College of Pharmacy.  Feb. 2011.