What is the meaning of false positives and false negatives in drug testing, and what commonly causes each?

• A. False positives indicate a drug is present when it is not, caused by cross-reactivity; False negatives miss a drug present, caused by cutoff levels, poor extraction, or assay interference.
• B. False positives are caused by instrument drift; false negatives by sample contamination
• C. False positives indicate a drug is present when it is not, caused by sample contamination; False negatives reflect high drug tolerance
• D. False positives and false negatives are random with no causes

Answer: (A)

In drug testing, a false positive means the test indicates a drug is present when it isn’t. This most often happens with screening immunoassays due to cross-reactivity, where substances that resemble the target drug or related compounds bind to the test antibodies and produce a signal even though the drug isn’t actually in the specimen.

A false negative means the drug is present but the test fails to detect it. Common causes include the drug concentration being below the assay’s cutoff level, poor extraction during sample preparation which reduces drug recovery, or assay interference that suppresses or masks the signal. Because of these limits, positive screening results are typically confirmed with a more specific method to verify the drug’s presence and concentration.