Lactulose is prescribed to treat hepatic encephalopathy. Which lab result would indicate the medication was effective?

• A. Bilirubin <1 mg/dL
• B. ALT 8 U/L
• C. Ammonia 16 mcg/dL
• D. AST 10 U/L

Answer: (C)

The key idea is that lactulose works by lowering the amount of ammonia that reaches the brain, so the lab that best shows this effect is a decreased serum ammonia level. In hepatic encephalopathy, ammonia produced by gut bacteria accumulates because the diseased liver can’t Detoxify it effectively. Lactulose isn’t absorbed; in the colon it’s fermented to acids that acidify the lumen. This converts ammonia (NH3) to ammonium (NH4+), which cannot be absorbed as easily and is excreted, and the laxative effect speeds stool passage, further reducing ammonia production and absorption.

An ammonia level of 16 mcg/dL is within the normal range (roughly 15–45 mcg/dL) and indicates that systemic ammonia has decreased, aligning with an effective response to lactulose. In contrast, bilirubin and liver enzymes like ALT and AST reflect liver cell injury or biliary function rather than the toxin-clearing action of lactulose, so they don’t directly measure improvement in hepatic encephalopathy from this therapy.