Should drug testing be considered screening tests?


Tests like cholesterol levels are considered screening tests because they are cost effective, have high sensitivities (a low rate of false negatives), and have been proven to be beneficial in the identification of conditions that may require treatment before symptoms occur. The vast majority of medical tests are not considered to be “screening” because they violate one or more of these principles, i.e. they are expensive, have lower sensitivities (higher rates of false negatives), lower specificities (higher rates of false positives), and/or have not been proven to be beneficial for testing large populations that do not have any signs or symptoms of the condition being tested for.

Urine or blood testing for illicit substances falls into the latter category. Even though they are inexpensive and easy to perform, drug testing should not be thought of nor used as a true screening test. For one thing, although drug testing is effective at helping to rule out illicit drug use, the problem comes from knowing how to interpret the results of a positive test. None of these tests are 100% specific and so the possibility of a false positive exists meaning that a positive drug test does not equal illicit drug use.

The problem of a false positive test is frequently encountered in the practice of medicine. Depending on the clinical circumstances and the nature of the initial test, follow up evaluation with more expensive and possibly more invasive testing is often required in order to verify the results. For example, an abnormality found on the chest x-ray of a smoker with a bad cough requires further evaluation. A CT scan of the chest, bronchoscopy, and even a needle biopsy to obtain a tissue sample for analysis are required before making a diagnosis of lung cancer and starting treatment.

However, the possibility of a false positive drug screen and the need for further testing and evaluation is rarely considered outside the context of clinical practice. Employers, school administrators, government agencies, and law enforcement can and do consider a positive drug test to be perfectly equivalent to an admission of illicit drug use. This frequently results in the administration of some form of punishment or corrective action being delivered without giving the accused the right to defend themselves in any way. Essentially, drug testing is an effective way to violate a person’s right to due process since most drug screening is managed by lay people in non-clinical roles who believe that drug testing is 100% reliable.  But this would be the same absurdity as giving chemotherapy to the smoker with the abnormal chest x-ray without first trying to verify the diagnosis with further evaluation (due process).

The other problem comes from the mass drug testing of large numbers of people (either random or at the initial point of contact). The interpretation of the results of a medical test are never as simple as positive or negative. The statistical probability of a false positive or a false negative result must be considered in concert with the pretest probability. The pretest probability is the likelihood that a patient really has the condition being tested for based on such things as risk factors for the disease, their prior history, and signs or symptoms of the disease. The pretest probability heavily influences a physician’s decision on whether or not to proceed with further testing or treatment. For example, the pretest probability for a pulmonary embolism (blood clot to the lungs) is going to be far higher for an 80-year-old heavy smoker with a history of advanced prostate cancer who presents with shortness of breath and blood tinged sputum then it will be for an 18-year-old non smoker with nasal congestion, a cough, and bad allergies and even though both patients can have clear chest x-rays. Essentially, doctors “profile” their patients.

Employers, school administrators, and government agencies fear being accused of discrimination far more than they worry about a person’s due process rights. So everyone is tested without regard to risk factors, a history of, or signs or symptoms of illicit drug use. Since the vast majority of any given general population do not use illicit drugs, their overall pretest probability for drug testing is going to be lower which means that any positive results have a higher chance of being a false positive. This principle was clearly demonstrated when a Pittsburgh woman was paid $143,500 to settle a lawsuit she filed after her 3 day old daughter was taken away by child protective services based on a drug screen that was positive for narcotics. It turns out that the hospital where she delivered her daughter routinely performs drug screens on every pregnant woman at delivery and all positive results are reported to the county CPS. The woman claims to have eaten a poppy seed bagel in the days before the test that caused the positive result. More importantly however, is the fact that there did not appear to be any reason to test this woman. By all accounts, no one suspected her of being a heroin addict.

This drug testing hysteria got it’s start in the 1980s alone with “just say no,” “this is your brain on drugs,” and the Drug Free Workplace Act of 1988. Originally testing was limited to high risk professions such as the transportation industry but it spread to everything else under the guise of combating lost productivity as a result of drug abuse. Basically, the Federal government expanded it’s monitoring of what you could put into your body to both at home and at work.

However, there is no hard proof that drug testing itself decreases drug use, improves safety, or increases productivity. Drug testing entities simply don’t realize that the vast majority of testing is not needed, is a waste of time and money, is not effective in reducing drug abuse, and may leave them at a higher liability risk when they don’t know how to deal with the possibility of a false positive.

Chris Rangel is an internal medicine physician who blogs at

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