A Stool pH & Reducing Substances test is a laboratory stool study that helps screen for carbohydrate malabsorption—most commonly problems digesting or absorbing sugars such as lactose (milk sugar), but also other carbohydrates (e.g., sucrose or glucose polymers). It is often used when diarrhea may be related to undigested sugars.

This test typically includes two related measurements:

1) Stool pH: This measures how acidic or alkaline the stool is. When carbohydrates are not absorbed in the small intestine, they pass into the colon where gut bacteria ferment them into acids (such as lactic acid and short-chain fatty acids). This can make the stool more acidic (a lower stool pH).

2) Reducing substances: This checks for the presence of unabsorbed sugars in stool. “Reducing substances” is a broad screening term that can detect sugars that may be spilling into the stool when digestion/absorption is incomplete. A positive result suggests that carbohydrates are present in the stool, which can be consistent with malabsorption or rapid intestinal transit.

What results can mean (plain language):

• Low stool pH (more acidic) and/or positive reducing substances can point toward carbohydrate malabsorption—often seen with lactose intolerance or other causes of poor digestion/absorption. This pattern may help explain symptoms like watery diarrhea, gas, bloating, and diaper rash in infants.

• Normal stool pH and negative reducing substances makes significant carbohydrate malabsorption less likely, though it does not completely rule it out. Your clinician may interpret results alongside symptoms, diet, and other tests (for example, a hydrogen breath test for lactose intolerance).

Because many factors can affect stool chemistry (diet, timing, bacterial activity, and how the sample is handled), this is considered a screening test. Abnormal results usually lead to follow-up evaluation rather than serving as a stand-alone diagnosis.

This test is commonly ordered when a clinician suspects that diarrhea or digestive symptoms are due to unabsorbed carbohydrates. It is used in both children and adults, and it is especially common in infants and young children with persistent diarrhea.

Your doctor may recommend stool pH and reducing substances testing if you have:

• Ongoing or recurrent watery diarrhea, particularly after meals or after milk/dairy intake

• Symptoms of lactose intolerance such as gas, bloating, abdominal cramps, and diarrhea after consuming milk, ice cream, or other dairy

• Poor weight gain or weight loss, signs of malnutrition, or concerns about nutrient absorption (malabsorption)

• Diarrhea in infants/children, especially if stools are frequent, watery, and associated with diaper rash or irritability

• Suspected post-infectious intolerance (temporary lactose malabsorption can occur after viral or bacterial gastroenteritis)

• Suspected intestinal injury or inflammation affecting the small intestine lining (where sugars are absorbed)

• Monitoring diet trials (for example, checking whether symptoms improve with lactose-free or reduced-carbohydrate diets, as directed by a clinician)

In practice, clinicians use this test to help distinguish carbohydrate-related diarrhea from other causes (such as infection, inflammatory bowel disease, or pancreatic insufficiency), and to decide whether additional testing—like a stool culture, ova and parasite exam, celiac testing, or a breath hydrogen test—may be needed.

• Lactose intolerance (lactase deficiency)
• Secondary lactose malabsorption after gastroenteritis (post-infectious)
• Celiac disease (can reduce carbohydrate absorption by damaging the small intestinal lining)
• Short bowel syndrome or other causes of reduced absorptive surface area
• Congenital carbohydrate malabsorption (rare; e.g., congenital lactase deficiency, glucose-galactose malabsorption)
• Disaccharidase deficiencies (e.g., sucrase-isomaltase deficiency)
• Small intestinal mucosal injury (from inflammation or other insults that impair absorption)

• Identifying whether dietary sugars/carbohydrates are contributing to chronic or recurrent diarrhea
• Supporting evaluation and management of suspected lactose intolerance (including guiding a lactose-free trial when appropriate)
• Assessing possible malabsorption in people with unexplained weight loss, poor growth, or nutrient concerns
• Helping parents and clinicians manage infant/toddler diarrhea and feeding intolerance
• Monitoring digestive health when symptoms suggest carbohydrate intolerance during illness recovery (e.g., after stomach flu)
• Providing clues that may prompt further workup for small intestine conditions (such as celiac disease) when clinically indicated