Why do you test positive on a urine but negative blood test?

There can be several reasons why a urine test may show a positive result for a substance while a blood test for the same substance comes back negative. Here are some possible explanations:

1. Timing: The timing of the tests can play a role. Urine tests can detect substances longer than blood tests. For example, certain drugs may remain detectable in urine for several days after they have been eliminated from the bloodstream.

2. Metabolism: The body metabolizes and eliminates substances differently. Some drugs are primarily excreted through urine, while others are broken down and removed through other routes, such as the liver. This difference in metabolism can lead to a positive urine test but a negative blood test.

3. Concentration: Urine tests are typically more concentrated than blood tests. This means that a small amount of a substance in urine may be more easily detected than the same amount in a larger volume of blood.

4. Hydration: Hydration levels can affect the concentration of substances in urine. Dilute urine may have lower concentrations of certain substances, potentially resulting in a negative test result. Conversely, concentrated urine may contain higher concentrations, increasing the likelihood of a positive test.

5. Contamination: Urine samples may become contaminated during the collection process, leading to false positive results. External substances or improper handling can introduce substances into the urine, affecting its accuracy.

6. Substance specificity: Some urine tests may be more sensitive and specific for certain substances compared to blood tests. This variation can lead to discrepancies in the results, where one test detects the substance while the other does not.

7. Individual factors: Individual factors such as health conditions, diet, and medications can affect the metabolism and excretion of substances. These factors may contribute to different results in urine and blood tests.

In cases where there is a discrepancy between urine and blood test results, it is essential to consult with a healthcare professional. They can interpret the results, consider the context of the tests, and perform further evaluations if necessary to determine the accurate presence and levels of the tested substance.