An infant with diarrhea has a positive copper reduction test and a stool pH of 5.0. Which conclusion is most appropriate?

• A. Further tests are indicated.
• B. Results are inconsistent—repeat both tests.
• C. The diarrhea is not due to carbohydrate intolerance.
• D. The tests provided no useful information.

Answer: (A)

The key idea is that this combination points to carbohydrate malabsorption as the cause of the diarrhea. A positive copper reduction test shows reducing substances in the stool, meaning unabsorbed sugars are present. An acidic stool pH (5.0) supports fermentation of those sugars by colonic bacteria, producing acids and lowering the pH. Together, they indicate carbohydrate intolerance, most often lactose intolerance in infants, but they do not identify exactly which carbohydrate is not being absorbed or how severe the intolerance is. That’s why further tests are warranted to pinpoint the specific carbohydrate involved and to guide treatment, such as dietary modification or a targeted diagnostic workup. The results do provide useful information; they just don’t complete the diagnosis on their own.