Lactose intolerance test

What Is a Food Intolerance?

Most food intolerances come down to the same mechanism: your body can’t properly break down a specific substance because it’s missing the digestive enzyme for the job. Lactose, the sugar in milk, needs the enzyme lactase. Without enough of it, lactose passes through the small intestine undigested and ferments in the colon, which is what causes the gas, bloating, and diarrhea associated with lactose intolerance [3]. If you suspect a different trigger food, our general food intolerance test page can help you figure out where to start.

How to Test for Lactose Intolerance: A Step-by-Step Guide

If you think dairy might be behind your symptoms, here’s a practical order to work through:

1. Track your symptoms. Note what you ate, how much, and when the bloating, gas, or diarrhea started. Symptoms from lactose usually show up within a few hours [3].
2. Check your family history. A parent or sibling with dairy troubles raises the odds that you have it too [2].
3. Try a short elimination phase. Cut dairy for one to two weeks and see if symptoms ease: this is the basis of a dairy intolerance test (more on that below).
4. See your doctor. They’ll do a physical exam and rule out look-alike conditions such as IBS or celiac disease before testing [1].
5. Get the right lab test for your age. Adults are usually offered a hydrogen breath test or a lactose tolerance (blood) test; infants and young children are more often given a stool acidity test [1][5].
6. Talk through your options. If lactose is confirmed, a dietitian can help you figure out what you can still eat and whether a lactase supplement fits your routine.

Lactose Tolerance Test (Blood Test): How It Works

The lactose tolerance test is the original blood-based method for diagnosing lactose intolerance, though it’s used less often today than the breath test. You fast overnight, then drink a liquid with a set amount of lactose. Over the next couple of hours, your blood is drawn several times to see how much glucose it releases as the lactose is digested [6].

How to interpret a lactose tolerance test: if your blood glucose rises well above your fasting level, your body is digesting the lactose normally. If glucose stays roughly flat, your body likely isn’t breaking the lactose down, which points to lactose intolerance [6]. There isn’t one universal cutoff number used everywhere, so ask your doctor how your specific lab defines a normal versus low response.

Hydrogen Breath Test: What to Expect

The hydrogen breath test is the test doctors reach for most often [1]. After fasting overnight, you drink a liquid containing a known amount of lactose. Every 30 minutes or so over the next few hours, you breathe into a device that measures hydrogen levels [1]. Normally, only a small amount of hydrogen shows up in your breath. If lactose isn’t being digested, gut bacteria ferment it and release extra hydrogen, which the device picks up [1][5]. A rise of more than 20 parts per million above your baseline, especially alongside worsening symptoms during the test, is generally read as a positive result for lactose intolerance [5].

Stool Acidity Test for Infants and Young Children

Babies and young children usually can’t do a hydrogen breath test properly, so doctors turn to a stool acidity test instead. A small amount of lactose is given, then a stool sample is checked for acidity and for undigested sugars. Acidic stool with detectable sugars suggests the lactose isn’t being fully absorbed [7].

What Is the Most Accurate Test for Lactose Intolerance?

Among the tests actually used in everyday practice, the hydrogen breath test is generally considered the most reliable and is the one most clinicians rely on first [1][3]. A small intestinal biopsy measuring lactase activity directly is technically the most precise method, but it’s invasive and rarely necessary, so it’s reserved for unusual or unclear cases [3].

What Are the First Signs of Being Lactose Intolerant?

The first signs usually show up within a few hours of having dairy, and commonly include:

• Bloating and a swollen feeling in the abdomen
• Gas
• Stomach cramps or pain
• Diarrhea
• Nausea, in some cases

How noticeable these are usually depends on how much lactose you’ve had and how little lactase your body is producing [3].

What to Do If You’re Lactose Intolerant?

Once you know dairy is the trigger, the next question is what to do about it. Cutting out dairy completely can mean missing out on nutrients it provides, like calcium, vitamin B12, and protein. Two practical routes help most people keep eating normally:

• Get familiar with where lactose hides in food so you can make informed swaps rather than cutting dairy blindly.
• Use a lactase enzyme supplement to help digest the lactose that’s already in your meal.

Lactase Supplements: Capsules, Drops, and Probiotics

Supplementing with the lactase enzyme helps reduce occasional gas, bloating, and discomfort associated with digesting dairy*. The three most common formats:

• Lactase capsules or tablets, taken right before or with your first bite of a dairy-containing meal. In a placebo-controlled crossover trial, orally supplemented lactase measurably reduced both symptoms and hydrogen breath levels compared with placebo [8].
• Lactase drops, stirred into milk or another non-acidic dairy drink ahead of time rather than taken with a meal.
• Lactase-producing probiotics, taken once daily, which introduce probiotic strains that generate lactase as they work in the gut.

*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Still unsure where to start? Our dietitians are happy to help you think it through, and you can also use our food intolerance test to check whether something other than lactose might be behind your symptoms.

If your symptoms persist, you’re losing weight without trying, or you find yourself cutting out more and more foods, talk to your doctor or a dietitian.

Sources

• 1. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Diagnosis of Lactose Intolerance [Internet]. Bethesda (MD): National Institutes of Health; 2018 [cited 2026 Sep 15]. Available from: https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance/diagnosis
• 2. NIDDK. Symptoms & Causes of Lactose Intolerance [Internet]. [cited 2026 Sep 15]. Available from: https://www.niddk.nih.gov/health-information/digestive-diseases/lactose-intolerance/symptoms-causes
• 3. Micic D, Rao VL, Rubin DT. Clinical Approach to Lactose Intolerance. JAMA. 2019;322(16):1600-1601.
• 4. Enattah NS, Sahi T, Savilahti E, Terwilliger JD, Peltonen L, Järvelä I. Identification of a variant associated with adult-type hypolactasia. Nat Genet. 2002;30(2):233-237.
• 5. De Geyter C, Van de Maele K, Hauser B, Vandenplas Y. Hydrogen and Methane Breath Test in the Diagnosis of Lactose Intolerance. Nutrients. 2021;13(9):3261.
• 6. Malik TF, Panuganti KK. Lactose Intolerance. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025.
• 7. American Academy of Pediatrics. Lactose Intolerance in Infants & Children: Parent FAQs [Internet]. HealthyChildren.org [cited 2026 Sep 15]. Available from: https://www.healthychildren.org/English/healthy-living/nutrition/Pages/Lactose-Intolerance-in-Children.aspx
• 8. Baijal R, Praveen Kumar HR, Yadav DP, et al. Effect of lactase on symptoms and hydrogen breath levels in lactose intolerance: A crossover placebo-controlled study. JGH Open. 2021;5(2):272-277.