Which method is used for confirmation after a positive immunoassay screening for barbiturate abuse in urine?

• A. nephelometry
• B. thin layer chromatography
• C. gas chromatography/mass spectrometry
• D. ultraviolet absorption spectroscopy

Answer: (C)

Screening tests like immunoassays are designed to be fast and sensitive, but their antibodies can react with similar compounds, which can yield false positives. To confirm a positive result, you need a method that can specifically identify the exact barbiturate present and accurately measure its amount. Gas chromatography paired with mass spectrometry does just that: the urine is first separated into its components by gas chromatography, and then the mass spectrometer analyzes the fragmentation pattern to confirm the identity of the barbiturate and determine its concentration. This combination is highly specific and can distinguish among different barbiturate compounds, which immunoassays cannot reliably do.

Other methods lack either the separation step or the specificity required for confirmation. Nephelometry is used for certain protein/antibody tests, not for drug confirmation. Thin layer chromatography is less sensitive and less specific, making reliable confirmation difficult. Ultraviolet absorption spectroscopy is non-specific in complex biological samples and cannot uniquely identify which barbiturate is present.