Drug Detection Cutoffs: What You Need to Know
A drug detection cutoff level is a critical part of a workplace testing program.
There are many decisions that have a critical impact on a workplace drug testing program including sample choice, frequency of testing, methodologies, and protocols for screening and confirmation. Urine is most commonly used for employee testing and is the only method approved for federally mandated drug testing. It provides a convenient and flexible method of workplace testing. (Learn more in DOT vs. Non DOT Testing: What's the Difference?)
When establishing a drug testing program and developing your workplace drug and alcohol policy, it's important to have a clear and consistent policy concerning the correct test panels and testing procedures. This policy must be based on a clear understanding of what the test detects and its accuracy. Drug tests may identify the substance of abuse or may identify a metabolite. The window of detection for different substances varies widely. In order to be effective, workplace drug testing programs must also guard against adulteration and contamination of samples. One of the most important parts of a workplace testing program is the drug detection cutoff level.
What's a Drug Detection Cutoff Level?
When a specimen is evaluated in the clinical lab, the results are reported quantitatively, in units of measurement such as nanograms per milliliter. The results of a screening test are reported to the employer as presumed positive or negative, but this is partially dependent upon the drug detection cutoff level. The drug detection cutoff level is a minimum measurement applied to a drug test, so that only traces of a drug or metabolite above a certain level is reported as positive. The cut-off level varies for each drug in the panel and takes into consideration passive exposures that can result in low levels of drug or metabolite.
The cutoff level will also vary depending upon the type of biological specimen used to test for the presence of a drug or drug metabolite. If the level of substance identified in the specimen is below a preset number, it will be reported as a negative test and if the level of substance is higher than the drug detection cutoff level, it is reported as presumed positive.
A negative drug screen does not mean the tested substance was not found, as it may have been present in levels too low to be reported. There are strong arguments against reporting all tests with even tiny amounts of substance as positive. The ramifications of having a presumed positive drug test can be damaging to employees and caution is necessary to avoid errors when employees may simply have had passive exposure or cross-reactivity to another substance.
The Detection Cutoff Level and It's Effect of Test Sensitivity
Drug detection cutoff levels indicate a threshold that is set to identify 100% of true-positive results and 95-98 percent of true-negative results, functions of the sensitivity and specificity of the test. Sensitivity and specificity are characteristics inherent to a test. They reflect reliability, which is the likelihood of a test to detect a substance that it is designed to detect and to give a negative result only when the substance is not present in the specimen. Together, sensitivity and specificity determine the accuracy of a laboratory test. They are interdependent characteristics.
Four results are possible when testing a specimen for a drug or metabolite. A true positive is a test that correctly identifies the substance and a false positive incorrectly detects the presence of the substance when none is present. A true negative confirms the absence of the substance and a false negative fails to detect the presence of the substance.
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Sensitivity refers to the proportion of people who use the substance in question who are identified by the test. Specificity is the proportion of people accurately identified as not using the substance. Tests with high sensitivity will have a high rate of “true positives” and a low rate of “false negatives.” Tests with high specificity will have a low rate of “false positives” and a high rate of “true negatives.” These characteristics of the test will determine the predictive value, or how likely it is that people with a positive test actually use the substance and how likely it is that people with a negative test do not use the substance.
A test with a low cutoff level will identify more positive specimens, but there will be more false positives among the group as a result of passive exposure. A test with a high cutoff level will identify fewer false positives, but it may fail to identify people who use the substance (false negatives.) If the cutoff level is high for a screening test is high, then the sensitivity is reduced. Some individuals with low levels of the substance will have a negative test result. However, if the cutoff level is too low for a screening test, an unacceptable number of false positive tests may result from passive exposure or cross-reactivity to other substances.
A Two-Tiered System Is the Standard for Workplace Testing
Workplace testing typically begins with an initial screening test that is designed to have a high sensitivity, while minimizing false positive results. The screening test is a way to exclude a group of negative specimens, but it is not a guarantee that the drug is not present in the specimen. A person with a negative screening test may have very small amounts of a drug or metabolite that fall below the drug detection cutoff level. In all cases, if the level in a specimen is above the cutoff mandated by the employer, the test is returned as presumed positive. All presumed positive results must be confirmed by a second tier confirmatory test.
Mandatory federal workplace drug-testing guidelines were established for federal employees with initiation of the Federal Drug-Free Workplace Program. Provisions for drug testing programs in the Federal sector were outlined in 1987 and include cutoff concentrations that are generally lower than those recommended for clinical practice. Federal drug detection thresholds are high enough to detect drug concentrations that are consistent with abuse, but may not detect medications at therapeutic concentrations. (Learn more in "8 Things Employers Should Know About DOT Drug and Alcohol Testing".)
Private-sector employers can establish their own drug-free workplace programs and establish testing guidelines, if they do not fall under Federal mandatory guidelines but are still limited by state regulations in some cases. (Learn more in "State Drug Testing Laws: What Should Employers Know?") Although cutoff levels in clinical practice may vary, laboratories and point of care tests for workplace purposes use Federal mandatory guidelines and SAMSHA recommendations for testing cutoff concentrations.
SAMSHA recommends relatively high cutoff levels to avoid the hazard of taking action against an employee who has had some sort of passive or therapeutic exposure. For example, a very low cutoff level for opioid metabolites may result in a positive test in a person with passive exposure from ingestion of poppy seed baked goods, which can result in low levels of opioid metabolites.
What is a confirmatory test and why is it necessary?
A confirmatory test is usually performed in the case of a presumed positive result from a screening test. All positive initial screening test results in Federal workplace testing must be followed by a confirmatory test, which has a high sensitivity and specificity for detection of individual substances. The confirmatory test cutoff level is set to confirm more than 95% of screening specimens with a positive result. Although this results in some false negative results, false positives results are less likely.
Although screening tests may not distinguish between drugs within a class, confirmatory tests detect individual compounds by advanced analytic techniques, comparing results to a reference standard based on a known compound. This allows accurate identification of illegal drugs like heroin, which react differently than prescription narcotics. A confirmatory test for heroin metabolites would not be positive through passive exposure to poppy seeds or in the case of therapeutic use of an opioid pain medication. (Learn more in "POCT vs. Lab Testing: What is the Difference?")
A licensed physician, known as a Medical Review Officer, is responsible for receipt and evaluation of drug test results. The MRO provides an independent, confidential review of the process and clarifies the results that are returned as positive, invalid, substituted, or adulterated.
Additional guidance is available on drug detection cutoffs
Most drug testing is performed in the workplace. Workplace drug testing falls into three categories, including federally regulated drug testing for certain Federal employees; federally regulated for non-Federal employees in safety sensitive positions; and nonregulated workplace testing for non-Federal employees. In addition to Federal guidelines for workplace drug testing, there are also a number of state laws and regulations that affect when, where, and how employers can implement drug-free workplace programs. (Learn more in "Medical Marijuana Law Differences and Contradictions".)