When taking a drug test, the color of the lines that appear on the test strip provides important information about the results. A blue line on a drug test is typically an indication that the test detected the drug or drugs that it was screening for. However, there are some nuances to interpreting drug test results, especially when multiple colored lines are present.
Blue Line Means Positive Result
Most drug tests will have a control line and a test line on the strip. The control line appears whether the test is positive or negative and ensures the test is working properly. The test line only appears if the drug metabolites being screened for are detected in the urine sample.
On many drug tests, a blue test line will appear if the drug is present in the urine. The intensity of the blue color is proportional to the amount of drug metabolite detected – the darker the blue, the more metabolite is present. A faint blue test line can indicate a positive result that is near the cutoff level for detection. An intense dark blue line signals a strongly positive sample.
So in summary, a blue test line, regardless of the shade, means the urine sample contains the drug or metabolite the test is intended to detect. The specific drugs that turn drug tests blue vary depending on what the test is designed for, but may include:
- Methamphetamines (meth, speed, crank, etc.)
- Amphetamines (Adderall, Dexedrine, etc.)
- Marijuana (THC)
- Opiates (heroin, morphine, codeine)
- Phencyclidine (PCP)
- MDMA (ecstasy)
- Tricyclic antidepressants
- Benzodiazepines (Valium, Xanax, etc.)
So in short, a blue test line indicates the drug test detected the substance it was screening for in the urine sample. The shade of blue relates to the concentration level.
Blue Line Plus Control Line = Positive
Many drug tests will have a control line appear whether the test is positive or negative. This serves as a way to validate the test reagents are working properly.
If a blue test line and a control line both appear, this indicates a positive drug test. The blue test line signals the drug metabolite was detected, while the control line shows the test is functioning correctly.
Sometimes, very faint blue test lines can be difficult to interpret. But if both the faint blue test line and dark control line appear, the test should be considered positive. The blue test line does not need to be dark or intense to be called a positive result.
No Blue Line Present = Negative Result
If no blue test line appears on the test strip, this typically indicates a negative result. With a negative drug test, you will generally only see the control line show up.
A negative result means the concentration of drug metabolite in the urine sample was below the cutoff level that the test can detect. So if you only see a control line appear, you can assume the person being tested was clean for the drug or drugs screened.
There are some situations where a drug test may be deemed invalid or inconclusive:
- The control line fails to appear. This could mean the test malfunctioned.
- A faint blue test line appears but no control line shows up. This makes it unclear if the blue line is real or an error.
- Multiple colored lines appear. Some tests have multi-colored lines for different drugs, so odd colors may be unclear.
- The color and intensity of the lines change as time passes. Consistent line colors are needed for clear results.
In these ambiguous cases, the best course of action is to have the person re-tested with a new drug test kit to obtain clear results.
What Level of Drug Causes a Positive?
A blue line will show up on a drug test if the drug metabolite concentration is above the established cutoff level. Here are some typical cutoff levels that must be exceeded to turn drug tests positive:
|Marijuana (THC)||50 ng/mL|
|Phencyclidine (PCP)||25 ng/mL|
These cutoff concentrations can vary slightly between testing laboratories but represent the approximate levels required for a positive drug test. Even faint blue lines can qualify as positive if above the cutoff.
How Long After Taking Drugs Will Tests Turn Blue?
How quickly a drug test turns blue after substance use depends on several factors:
- Drug detection window – This is how long the metabolites remain detectable in urine. For marijuana, opiates, meth, and other drugs this can be 1-4 days or longer after last use.
- Individual variation – People metabolize drugs at different rates, leading to different detection windows.
- Quantity consumed – Larger doses lead to more metabolite excretion and longer detection times.
- Frequency of use – Frequent or heavy users store drug metabolites in fat cells, extending detection periods.
- Fluid intake – Dilute urine can lower metabolite concentration below the cutoff threshold.
- Test sensitivity – Some tests have lower cutoff levels and can detect smaller amounts of metabolites.
As a general timeline, a drug test may turn blue anywhere from 1-4 days after last drug use for most people. But chronic heavy users can have positives for a week or longer after quitting.
False Positives – Other Reasons for Blue
While rare, some circumstances can cause a drug test to show up as a false positive blue line:
- **Contaminated sample** – Another person’s urine with drugs gets in the sample.
- **Mix-up at the lab** – The sample gets swapped or mislabeled.
- **Passive inhalation** – Exposure to secondhand marijuana smoke.
- **Poppy seeds** – Eating poppy seeds can cause opiate positive.
- **Medications** – Some over-the-counter meds can trigger false positives.
- **Medical conditions** – Kidney disease and other illnesses can affect results.
A false positive result can have serious consequences for the person being tested. Confirmatory secondary testing is recommended whenever unexpected positive results occur.
Blue Dye Tests vs. Blue Lines
Some urine drug tests use a blue dye indicator that mixes with urine to indicate the presence of specific drugs. These tests function differently than the blue line on test strips.
Blue dye tests typically rely on adulteration strips that detect abnormal pH, gravity, or other sample qualities. If drugs are present in the urine, it can change the dye color from green to blue.
Blue line tests are more common and rely on competitive binding and antibodies to detect actual drug metabolites, not just general abnormalities. Both approaches signal drug-positive urines when blue color appears.
A blue line on a urine drug test essentially always means the test detected the drug or metabolite it was designed to screen for. The intensity of the blue color corresponds to the concentration of drug metabolite present in the sample. Even very faint blue lines can qualify as a positive.
However, sometimes a blue line can appear falsely due to contamination, mix-ups, or medical causes. Confirmation testing is recommended when false positives are suspected. But in general, the appearance of any blue line during drug testing means the urine sample tested positive for the particular drug or drugs.