March 21, 2018: What causes false positives or false negatives on the Urine Drug Screen (UDS)?

We frequently get questions regarding the meaning of false positives and negatives on the urine drug screen.  I’ve asked a real expert to address them.  ds

On false positives:
In a false positive urine drug test, the drug of interest is not present in the sample.  False positives can be due to unrelated cross-reacting compounds (e.g. sulfamethoxazole/trimethoprim can cause a false positive buprenorphine screen) or due to the presence of related compounds that are not confirmed (some designer amphetamines will cause a positive amphetamines screen but will not be confirmed).  Because of the potential for false positives laboratories will report screening results as “presumptive” positive.  Until the laboratory has reported confirmatory test results, presumptive positive screens should be regarded as preliminary and interpreted with caution.

On false negatives:
In a false negative urine drug test, the drug of interest is present in the sample but is not detected.  False negatives can occur when the concentration of drug is below the threshold for positivity (also called the cutoff).  False negatives can also occur due to varying cross-reactivity to drugs that belong to one class.  For example, benzodiazepines assays are prone to false negatives for lorazepam and clonazepam because antibodies used in the test have lower sensitivity for these drugs compared to diazepam and alprazolam. 

Below is a summary of known false positives and false negatives for VUMC urine drug screens.  This list is subject to change and the laboratory is always available to assist with interpretations (call 5-LABS).


** Synthetic opioids like tramadol, tapentadol, and fentanyl are not detected by this assay.
-- False negative/positive not frequently observed using VUMC’s test.

This Question was prepared by: Jen Colby PhD, Chief Toxicology Lab; Associate Director of Clinical Chemistry VUMC.

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Donna Seger, MD
Medical Director
Tennessee Poison Center
Poison Help Hotline: 1-800-222-1222