Abnormal stool composition Elevated stool reducing substances 2025-08-12T07:34:50Z Elevated fecal reducing substances Increased reducing substances in stool Elevated level of reducing substances in the feces. If carbohydrate malabsorption is suspected, a reducing substance should be investigated in the stool using the Benedict or Fehling test. The presence of reducing substances may indicate an abnormality in carbohydrate (e.g., sucrose, lactase) metabolism. The unabsorbed sugars in stool are measured as reducing substances.