Drug tests can be performed on blood, hair, saliva, urine, and nails. Urine is frequently used, as it is easy to collect, and drugs and their metabolites are concentrated in the urine, allowing for a prolonged period of detection.There are two main types of urine drug tests: immunoassays and gas chromatography mass spectrometry (GC-MS). Immunoassays work by using antibodies to check if the original drug or its breakdown products are present. The antibody is designed to bind only to a specifically shaped molecule. These tests can be run quickly and inexpensively. They are the tests most often used in employment and medical settings. GC-MS is a method that identifies a specific compound within a sample, like a fingerprint. It is more accurate than immunoassays, but much more expensive and time consuming to run. It is often used as a confirmatory test after a positive immunoassay.
What are the problems with these tests?
False positives are a well-known problem with immunoassay based urine drug tests. The antibody used in these tests is designed to bind to a specific molecule shape. If there is another drug or substance that has a similar shape, it may be able to bind to the antibody. Therefore, the test will be positive, but falsely, as the antibody did not bind the drug in question, but something with a similar shape. The false positives for each urine drug test are different and must be considered when interpreting a result. False negatives are another common problem. A false negative is when the drug of concern has been used, but the test does not detect it. This can occur for a variety of reasons - the test, the drug, and the patient. Some variables include when the drug was ingested, how much of the drug was ingested, what the cut-off for detection is for the test, the concentration of the urine, and if there were substances added to the urine to tamper with the test.
How long will a urine drug test stay positive?
There are many factors that affect how long a urine drugs test will stay positive. A few examples include frequency of drug use (short term versus long term), the specific drug used, whether the drug accumulates in the body, and if there are metabolites of the drug that are detected by the test.
Urine drug tests are complex, but understanding these fundamentals can help to simplify them. Caution should be used when interpreting any urine drug tests, and it should only be done by a medical professional with the appropriate knowledge and experience.
Navneet Cheema, MD