How is lactose intolerance diagnosed?

Lactose intolerance can be diagnosed through several clinically validated methods, each with distinct advantages and limitations. The most commonly used and evidence-based diagnostic tests include the hydrogen breath test, lactose tolerance test, and stool acidity test—particularly in infants and young children. The hydrogen breath test is considered the gold standard: after an overnight fast, the patient consumes a standardized lactose solution (typically 25–50 g), and breath samples are collected at regular intervals over 2–3 hours to measure hydrogen concentration. A rise of ≥20 parts per million (ppm) above baseline indicates malabsorption due to lactase deficiency. The lactose tolerance test measures serial blood glucose levels before and after lactose ingestion; a rise of less than 20 mg/dL suggests impaired lactose digestion. In infants, the stool acidity test detects lactic acid and other short-chain fatty acids in stool samples, as undigested lactose undergoes bacterial fermentation in the colon. Genetic testing for the LCT gene polymorphism (e.g., C/T-13910 variant) may support a diagnosis of primary (adult-type) lactase deficiency but does not assess functional enzyme activity and is not routinely recommended for clinical diagnosis. Importantly, symptom-based self-diagnosis or unstructured dietary elimination is unreliable and may lead to unnecessary dietary restrictions; formal testing should be pursued under the guidance of a healthcare provider to rule out other conditions such as irritable bowel syndrome, celiac disease, or inflammatory bowel disease that mimic lactose intolerance.