Faint Lines Explained
When you see a faint line on a drug test...
We get many questions about faint lines. Although there is no single factor which leads to faint test lines, the following are examples of possible causes.
2. Cross-Reactivity: We live in an era of unprecedented use of prescription and over-the-counter medications. Although they may not produce the same effect in the body, some medications are chemically similar to the drugs detected in a rapid drug screen test. If a compound is similar enough in structure to the drug being tested, it is possible that the presence of that compound could lead to faint test lines or even a false positive drug screen result. This phenomenon is one of the primary reasons why presumptive positive drug screen test results should be confirmed by an alternate methodology.
3. Assay-to-assay differences: In drug testing, it’s common practice to screen for multiple drugs simultaneously. In the most sophisticated products, as many as 12 drugs can be identified by a single drug screen device. Because of this, it’s easy to forget that each individual assay (even an individual test line on a multi-drug strip) is a separate diagnostic test with unique characteristics. For instance, some rapid drug assays are naturally lighter in line intensity than others. Most often, the light-line phenomenon is seen in THC metabolite tests although drug-class assays such as Benzodiazepines or Tricyclic Antidepressants can also be affected. As with the “matrix effect,” as a result of this possible root cause for light test lines, it is important to remember that a “line is a line” regardless of how faint and, the presence of a test line within the time limit of the test indicates a negative result for that assay.
4. Trace levels of drug in the specimen: It is possible that a faint test line on a rapid drug screen test is the result of the presence of “some” of the drug in the urine specimen (particularly in the case of THC). If this is the case, it is still critically important to note that the concentration of the drug is very likely below the cutoff level of the test, which indicates a negative result. Given the number of other factors which could cause faint test lines, it’s best to not over-interpret light test lines as containing some drug and just remember that the presence of a test line indicates a negative result for that drug.