Doctors usually test for lactose intolerance with a hydrogen breath test or a lactose tolerance blood test, and sometimes with stool testing in infants and small elimination “trials” with diet changes.
Main medical tests
- Hydrogen breath test: After fasting, you drink a lactose-containing liquid, then breathe into a collection bag or device at set intervals; a rise in hydrogen in your breath shows that undigested lactose is being fermented by gut bacteria, which suggests lactose intolerance. This is the most commonly used test because it is noninvasive and fairly accurate.
- Lactose tolerance blood test: You fast, have baseline blood drawn, then drink a lactose drink and have blood sugar checked over a couple of hours; if your blood glucose does not rise as expected, it suggests your body is not breaking lactose down properly. This test is less commonly used than the breath test but can still confirm the diagnosis.
Tests in babies and kids
- Stool acidity test: In infants and young children, undigested lactose produces lactic acid and other acids in the stool, so a more acidic stool (often with reducing sugars) suggests lactose intolerance. This is helpful when breath or blood tests are hard to perform.
At‑home and clinical “trials”
- Diet trial/elimination test: A doctor may have you avoid lactose for a week or two and see if symptoms such as gas, bloating, and diarrhea improve, then reintroduce lactose to see if symptoms return. There are also structured at‑home “tests” where you consume a measured lactose-containing food and track symptoms, but these do not replace formal medical testing.
If you suspect lactose intolerance, it is important to talk with a clinician, both to choose the right test and to rule out other conditions that can mimic it (such as celiac disease, IBS, or inflammatory bowel disease).
