A drug screen is a technical analysis of a biological specimen, most often urine, designed to detect the presence of specific parent drugs or their inactive metabolic byproducts. This analysis serves as an objective tool for monitoring compliance with policy or law, with primary applications found in pre-employment hiring, post-accident investigations, sports competition, and court-ordered monitoring programs. The interpretation of these results relies on a deep understanding of the testing methodology, which prioritizes accuracy and legal defensibility.
Screening Versus Confirmatory Testing
Drug testing protocols are designed as a two-step process to ensure the highest level of accuracy and prevent reporting a false positive result. The initial step is a laboratory-based immunoassay (IA) screen, which is fast and cost-effective. It uses antibodies to produce a presumptive positive result if a drug or its metabolite is present. Because the immunoassay is highly sensitive, it can sometimes react to substances with similar chemical structures, meaning it is only a preliminary indication.
Any specimen that yields a presumptive positive result must proceed to a second, more definitive analysis called confirmatory testing. This second test utilizes advanced technologies like Gas Chromatography/Mass Spectrometry (GC-MS) or Liquid Chromatography/Mass Spectrometry (LC-MS/MS), often referred to as the “gold standard” in forensic toxicology. These methods separate the specimen’s components and identify the exact molecular structure of the drug or metabolite, providing a precise and legally defensible quantitative measurement.
Interpreting Positive and Negative Results
The interpretation of a drug test result hinges on a predetermined concentration known as the “cutoff level,” which is expressed in nanograms per milliliter (ng/mL) of the specimen. A result is reported as negative if the concentration of the drug or its metabolite is below this established cutoff threshold. A negative result does not necessarily indicate the complete absence of a substance. For instance, an individual with a metabolite concentration of 40 ng/mL for marijuana would be reported as negative if the standard initial screening cutoff is set at 50 ng/mL.
A positive result is reported only when the concentration of the identified substance meets or exceeds the established cutoff level, both in the initial screen and the confirmatory test. These specific thresholds are set to minimize the risk of a positive result from passive exposure or trace amounts. They ensure the result indicates use above a non-permissible level.
What Invalid or Adulterated Results Mean
The integrity of the specimen itself must be verified, as not all drug screens result in a simple positive or negative finding.
Dilute Results
A result may be reported as Dilute if the specimen’s creatinine and specific gravity values are lower than expected for normal human urine. This can suggest excessive water consumption or an attempt to mask drug use.
Substituted Results
A specimen is reported as Substituted if the creatinine and specific gravity values are so far outside the normal physiological range that the sample is inconsistent with human urine.
Adulterated Results
The laboratory will report a result as Adulterated if chemical testing detects a substance that is not a normal constituent, such as bleach or nitrites. These substances were deliberately added to interfere with the testing process.
Invalid Results
An Invalid result is reported when an unidentified interfering substance or an abnormal physical characteristic prevents the laboratory from obtaining a valid result.
Findings of adulteration, substitution, or an invalid result frequently trigger a requirement for an immediate re-test under direct observation to ensure the integrity of the new sample.
The Role of the Medical Review Officer
The final step in the drug testing process for any result that is not a simple negative is the involvement of the Medical Review Officer (MRO). The MRO is a licensed physician with specialized training in substance abuse and toxicology. The MRO acts as an impartial gatekeeper, reviewing all laboratory results that are confirmed positive, substituted, adulterated, or invalid.
For a confirmed positive result, the MRO contacts the tested individual to conduct a confidential interview. This allows the individual to provide documentation for a valid prescription that justifies the drug’s presence in their system. If the MRO verifies that a valid prescription was used correctly, they report the final result to the employer or requesting entity as a verified negative. The MRO also evaluates presumptive positive results that may arise from cross-reactivity with common over-the-counter medications, ensuring that only illicit use or unapproved prescription drug use is reported as a final positive finding.