The stool pH and reducing substances test checks stool acidity and detects reducing sugars to identify carbohydrate malabsorption. Stool pH is measured with pH paper; reducing substances are detected by chemical tests. Acidic stool (low pH) with positive reducing substances suggests lactose or other carbohydrate malabsorption, often used in infants and children with chronic diarrhea, bloating, or failure to thrive.

Reducing substances in stool are sugars and other compounds that reduce copper ions in chemical tests, indicating unabsorbed carbohydrates in the gut. Their presence suggests carbohydrate malabsorption—common causes include lactase deficiency, congenital enzyme defects, small‑bowel mucosal disease or bacterial overgrowth—and typically produces acidic, watery stool. Detection uses simple bedside tests (e.g., Benedict’s/Clinitest) and guides further evaluation and dietary management.

To lower stool pH, promote bacterial fermentation by eating more soluble and fermentable fiber (oats, bananas, legumes, onions), resistant starch (cooled potatoes, rice), and probiotic foods (yogurt, kefir, fermented vegetables). Avoid frequent antacid overuse and unnecessary antibiotics. Maintain hydration and regular bowel habits. If you have ongoing GI symptoms or need personalized guidance, consult your healthcare provider before major diet or medication changes.

Normal stool is slightly acidic to neutral, typically around pH 6.5–7.5. pH varies with diet, gut bacteria and digestion. Persistent low pH (acidic stool) may suggest carbohydrate malabsorption or rapid transit, while unusually alkaline stool can be seen with some infections or bile/pancreatic disorders. See a healthcare professional if abnormal pH accompanies ongoing symptoms.

If a stool reducing substances test is negative, no abnormal amounts of reducing sugars are detected, suggesting normal carbohydrate digestion and absorption. This makes sugar malabsorption or lactose intolerance less likely as the cause of symptoms. Persistent diarrhea or other concerning signs merit further evaluation (stool cultures, ova/parasites, fecal fat, blood tests) and discussion with a clinician to find alternative causes.

A stool test can be positive for hidden (occult) blood indicating gastrointestinal bleeding, pathogenic bacteria (Salmonella, Shigella, E. coli, Clostridioides difficile), parasites and ova (Giardia, Entamoeba), viral antigens, and inflammatory markers or fecal leukocytes (calprotectin, lactoferrin) that suggest IBD. It may also show excess fat (steatorrhea) or specific pathogen DNA/RNA on PCR testing.