An In-Depth Look at the Accuracy of Drug Tests

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5 September 2023

An In-Depth Look at the Accuracy of Drug Tests

There is a lot of information available on the accuracy of drug tests, but it is frequently contradictory and often misleading. Your drug testing program is a substantial investment meant to protect your company and your people, so accuracy is critical. How accurate are drug tests? The answer depends on several factors including collection and testing procedures, the detection window for each drug and the type of test performed.

Urine Tests

Urine is the most frequently used test and is employed by 90% of workplace drug testing programs, including most major corporations and companies governed by Department of Transportation (DOT) regulations. Urine tests detect the presence of drug metabolites, the by-products created when your body metabolizes a drug. This type of testing can involve either a screen, called an immunoassay, or a test, or both. Essentially, the screen detects the presence of a drug but is considered less accurate and specific. If the results of the screen are positive, a test is then used to confirm the presence of the drug. Urine tests are popular but are also the tests most frequently targeted by people trying to cheat the system.

Saliva Tests

Saliva tests are used by 10% of employers and predict the presence of parent drugs. Parent drugs are the original drugs. For example, chlordiazepoxide is the parent drug of the benzodiazepine class of drugs. Saliva tests are both less expensive than blood tests and less invasive than either the urine or blood test. Because very little of certain drugs make their way into our saliva and the detection window is very narrow, these tests can only detect current and recent drug use. The accuracy of the test depends on the type of drug being tested but its ease of use and low cost make it an attractive test for some employers.

Hair Tests

Hair follicle drug testing is the test of choice for only about 7% of employers. This type of testing also tests for the presence of drug metabolites and is very thorough in that it can detect both the presence of a drug and patterns of use. Unfortunately, however, hair testing is the least accurate type of test. While it can detect long term use, it cannot detect recent use as hair takes time to grow and it is also not very useful in detecting sporadic use. Since even small amounts of powder or smoke can attach themselves to hair, it is also very easy for the specimen to become contaminated and for the test to produce a false positive. Interestingly, however, hair and saliva both have higher positive hit rates than urine tests.

Blood Tests

Blood tests are considered the most accurate form of testing, but they are also the most invasive and the most expensive. Consequently, they are seldom used in formal workplace drug testing. It is also important to note that since drugs do not remain in the bloodstream for long the window of opportunity for these tests is relatively small. Marijuana, for example, can disappear from the bloodstream in as little as a few hours for some users.

The accuracy of individual drug tests is measured by two related measures: sensitivity and specificity.

Measure of Accuracy: Sensitivity

The sensitivity of a test refers to how sensitive the test is to specific drugs or, in other words, how good it is at detecting drug use. The oft-quoted statistic that drug testing is 99% accurate or better usually refers to the sensitivity of the test. If a test has a high sensitivity you can be certain that if the result is negative, the employee is not abusing the drugs that were tested. The sensitivity of a drug test depends on the type of drug being tested. Cannabis, for example, tends to have a very high sensitivity.

Measure of Accuracy: Specificity

Specificity refers to how well the test identifies people who are not using drugs. It is expressed as the percentage of times the test can be relied on to correctly identify negative results. Both specificity and sensitivity are calculated from surveys of test results which are then plugged into a specific mathematical formula.

False Positives

False positives occur in approximately 10% of initial drug tests. Over the counter and prescription drugs as well as certain other substances can trigger a false positive. Fortunately, if an initial screen detects drug use, a second test called a Gas Chromatography-Mass Spectrometry (GC-MS) test is usually ordered to eliminate any false positives.

False Negatives

False negatives are of far more concern for employers, but they are generally very rare with modern tests and optimal laboratory and collection site conditions. As recreational marijuana use increases, there has been a suggestion that the inhalation of passive smoke could lead to a false negative, but this is not a valid argument as the concentrations required for a positive lab test are far higher than could be achieved through passive inhalation. Usually, false negatives are the result of an employee diluting a specimen or attempting to provide a fraudulent sample. Collection services usually have protocols in place to prevent this from happening.

Can you beat a drug test?

The short answer is no. The Internet abounds with ways to beat drug tests, from using a diluting agent such as water or juice to substituting freeze-dried or synthetic urine to using an oxidizing agent such as nitrate to break up the detectable metabolite. While many of these would work in theory, in practice, the processes and checks put in place by most professional labs and collection services ensure they seldom work. One of the ways they detect contamination is through validity screening. This essentially tests samples for temperature, gravity, density and pH levels but also screens for abnormal levels of creatine and the presence of altering oxidants. Additional precautions taken during collection and testing include:

  • Securing the test taker’s belongings separately
  • Screening for adulterants
  • Supervised observation
  • Securing all water sources
  • Enforcing hand washing

There are a variety of factors that affect the accuracy of drug testing, but it is a highly scientific process that is very accurate if samples are collected by trained professionals under tightly controlled circumstances and tested in reputable laboratories. (Learn more in 6 Up and Coming Drug Testing Technologies You Need To Know About).