In the Berthelot reaction, which contaminant will cause the urea level to be falsely elevated?

• A. Sodium fluoride
• B. Protein
• C. Ammonia
• D. Bacteria

Answer: (C)

In this assay, you’re measuring ammonia that comes from urea after it’s broken down by urease. The Berthelot reaction forms a blue compound from ammonia in alkaline conditions, so the color intensity reflects the amount of ammonia present. If there’s free ammonia already in the sample as a contaminant, it will contribute to the color signal regardless of how much urea is actually there, making the urea level appear falsely high. The other options don’t directly add ammonia to the reaction: sodium fluoride preserves and can inhibit bacterial enzymes, protein can cause nonspecific interference, and while bacteria could generate ammonia, the direct contaminant that causes the false elevation is ammonia itself.