Interpreting Drug Test Results

Result: Negative Specimen

A negative test result means that a particular substance was not detected at or above the cutoff concentration in the specimen. A negative drug test does not necessarily mean the patient has not used a particular substance or taken the prescribed medication.

Negative test results can occur if:

  • Errors were made in interpretation of the test.
  • The patient has induced enzyme levels from smoking or liver disease and eliminates the medication more rapidly than usual (e.g., methadone).
  • The patient has a shortened gastrointestinal tract from surgery and does not absorb the drug sufficiently for detection.
  • The patient ran out of medication.
  • The patient took the medication but not when expected or during the window of detection for the ordered test.
  • The patient was thirsty and drank sufficient water to dilute the specimen.
  • The patient may have consumed an excessive amount of fluids to deliberately dilute a urine specimen.
  • The appropriate test for a particular medication or substance was not performed.
  • The cutoff concentration used in the test was set too high, so small amounts of the drug/drug metabolites were missed.
  • The parent drug and/or its metabolites were excreted before specimen collection (e.g., outside the detection window).
  • The specimen may have been adulterated or substituted.

Result: Positive Specimen

A positive screening test result means that a particular substance was detected at or above the administrative cutoff concentration in the specimen. Confirmatory tests are frequently performed for specimens with positive screening results.

False-positive results are possible with screening (initial) tests. If a presumptive positive is confirmed by a second methodology, such as gas chromatography/ mass spectrometry (GC/MS) or liquid chromatography/mass spectrometry/ mass spectrometry (LC/MS/ MS), a false positive is highly unlikely if the test is performed correctly.

Result: Adulterated or Substituted Specimen

Urine is the easiest specimen to adulterate, and commercial formulas of synthetic urine are available for substitution. Other fluids, including water, also have been used for substitution. If the test result indicates that the specimen has been adulterated or substituted, the practitioner collects another specimen and reviews procedures to determine whether the temperature and pH of specimens are being checked immediately after collection.

Result: Dilute Specimen

A dilute urine specimen can be negative or positive, depending upon the degree of dilution and amount of drug excreted. If the test result shows that the specimen has been diluted, the practitioner should discuss both the dilution and the negative or positive test result with the patient.