The most common way to test for lactose intolerance is a hydrogen breath test, which measures gas in your breath after you drink a lactose solution. It’s noninvasive, takes about three to four hours, and is widely available through gastroenterology offices and hospitals. But it’s not the only option. Depending on your age, symptoms, and situation, your doctor may use a blood test, genetic test, stool test, or even a structured elimination diet to reach a diagnosis.
The Hydrogen Breath Test
This is the most widely used diagnostic test for lactose intolerance. The logic behind it is straightforward: when your body can’t break down lactose in the small intestine, the sugar passes into your colon, where gut bacteria ferment it and produce hydrogen gas. That gas gets absorbed into your bloodstream, travels to your lungs, and shows up in your breath.
During the test, you drink a liquid containing a measured dose of lactose. You then breathe into a collection bag roughly every 30 minutes for three to four hours. Each sample is analyzed for hydrogen levels. A rise of more than 20 parts per million (ppm) above your baseline reading is considered a positive result, meaning your body isn’t digesting lactose properly.
The test is reliable for most people, but certain things can throw off results. Bacterial overgrowth in the small intestine can produce hydrogen on its own, leading to a false positive. Transit time through the gut varies significantly from person to person, so an early spike in hydrogen can sometimes be misinterpreted. Recent antibiotic use can also suppress the bacteria responsible for producing hydrogen, potentially causing a false negative.
How to Prepare for Testing
Both the breath test and the blood test (described below) require the same basic preparation. You’ll need to fast for 8 to 12 hours before the test. If you’ve recently taken antibiotics, most providers want you to wait two to four weeks before testing, since antibiotics alter gut bacteria and can skew results. Avoid smoking or strenuous exercise the day before.
Certain medications can also interfere. Laxatives, antacids, and stool softeners may need to be paused beforehand, so let your provider know about everything you’re taking. If you’re having a breath test, you may be asked to brush your teeth or rinse your mouth right before the first sample to eliminate any bacteria in your mouth that could produce hydrogen on their own.
The Blood Glucose Test
The lactose tolerance blood test takes a different approach. Instead of measuring gas, it tracks what happens to your blood sugar after you drink lactose. Normally, lactase (the enzyme in your small intestine) breaks lactose into two simpler sugars, glucose and galactose, which then enter your bloodstream. If you’re lactose intolerant, that breakdown doesn’t happen efficiently, and your blood sugar won’t rise as expected.
After drinking the lactose solution, your blood is drawn at intervals over two hours. A blood glucose rise of more than 30 mg/dL is considered normal, meaning you’re digesting lactose fine. A rise of less than 20 mg/dL is abnormal and points toward lactose malabsorption. Anything between 20 and 30 mg/dL falls into a gray zone and may require further testing.
This test is used less often than the breath test because it’s slightly more invasive (multiple blood draws) and can be affected by other conditions that influence blood sugar, like diabetes.
Genetic Testing
A genetic test can tell you whether you carry the DNA variants associated with lifelong lactose digestion. The test looks at a regulatory region within a gene called MCM6, which controls how much lactase your body produces. People who carry certain variants in this region continue producing lactase into adulthood and can digest dairy without problems. People without those variants gradually lose lactase production as they age, which is the most common cause of lactose intolerance worldwide.
The test is typically done with a blood draw or cheek swab and requires no fasting or lactose challenge. It’s useful for confirming primary lactose intolerance, the inherited type that develops over time. But it won’t detect secondary lactose intolerance caused by gut damage from infections, celiac disease, or inflammatory bowel disease. In those cases, your genetics might look normal even though you currently can’t tolerate dairy.
Stool Acidity Test for Infants
Young children and infants can’t reliably blow into a bag on command, so doctors use a different method. The stool acidity test checks the pH of a child’s stool. When lactose goes undigested, bacteria in the colon ferment it and produce acids that lower the stool’s pH. A separate component of the test looks for the presence of glucose in the stool, which signals undigested lactose that has been partially broken down by bacteria. Together, these markers give pediatricians a noninvasive way to identify lactose malabsorption in very young patients.
Small Bowel Biopsy
This is the most direct but most invasive option. During an endoscopy, a small tissue sample is taken from the lining of your small intestine and analyzed in a lab to measure lactase activity directly. The test can also measure other digestive enzymes at the same time, which helps distinguish lactose intolerance from other conditions that damage the intestinal lining, like celiac disease. When the intestinal villi are broadly damaged, all brush border enzymes tend to be low. When only lactase is very low while other enzymes remain normal, that points specifically to lactose intolerance.
Because it requires sedation and an endoscopic procedure, a biopsy is rarely done solely to diagnose lactose intolerance. It’s typically performed when a patient is already undergoing endoscopy for another reason, such as investigating chronic diarrhea or suspected celiac disease, and lactase testing is added on.
The Elimination Diet Approach
Some people skip formal testing entirely and use a structured elimination diet instead. The idea is simple: remove all dairy from your diet for two to three weeks and track whether your symptoms improve. If bloating, gas, cramping, and diarrhea resolve during the elimination phase and return when you reintroduce dairy, that’s strong practical evidence of lactose intolerance.
For a more controlled version, you can do a “milk challenge” by reintroducing a glass of milk on an empty stomach after the elimination period and watching for symptoms over the next few hours. This informal approach won’t give you a number on a lab report, but for many people it provides a clear enough answer to guide dietary choices. It’s also entirely free and requires no medical appointment.
The limitation is that an elimination diet can’t distinguish between lactose intolerance and a milk protein sensitivity, since both improve when you remove dairy. If the distinction matters for your treatment, a formal test is the better route.
Which Test Makes the Most Sense
For most adults, the hydrogen breath test is the standard starting point. It’s noninvasive, widely available, and gives a clear numerical result. The blood glucose test serves as an alternative when breath testing isn’t available or when results are inconclusive. Genetic testing is most helpful when you want to know if your intolerance is the inherited, lifelong type or something triggered by another condition. For infants and toddlers, the stool acidity test is the go-to option. And a biopsy is reserved for complex cases where other diagnoses are on the table.
Many people with mild symptoms find that a two-to-three-week elimination diet gives them all the information they need without any formal testing. If your symptoms are severe, worsening, or accompanied by weight loss, a diagnostic test provides more precise information and helps rule out other conditions that mimic lactose intolerance.