The most reliable way to test for lactose intolerance is the hydrogen breath test, which measures how well your body breaks down lactose by tracking hydrogen levels in your breath over a few hours. A rise of 20 parts per million (ppm) or more above your baseline reading is considered a positive result. But there are several other testing options, from a simple at-home elimination diet to genetic testing, depending on your situation.
The Hydrogen Breath Test
This is the standard diagnostic test that most doctors will recommend. You drink a liquid containing a measured dose of lactose, then breathe into a collection device at regular intervals over two to three hours. If your body can’t properly digest lactose, bacteria in your colon ferment it and produce hydrogen gas, which enters your bloodstream and eventually shows up in your breath.
A normal result means hydrogen stays below 20 ppm above your fasting level. Anything at or above that 20 ppm threshold is considered positive for lactose malabsorption. You’ll need to fast overnight before the test, and your doctor may ask you to avoid certain foods and medications in the days leading up to it, since things like antibiotics and high-fiber meals can affect the results. The test itself is painless, though drinking that much lactose on an empty stomach can trigger the exact symptoms you’re trying to diagnose: bloating, cramping, gas, or diarrhea. That discomfort is temporary and actually serves as additional evidence alongside the breath readings.
The Elimination Diet Approach
If you want to test yourself before seeing a doctor, an elimination diet is the simplest starting point. Remove all dairy products and other sources of lactose from your diet for two to four weeks. This means checking labels carefully, since lactose hides in bread, processed meats, salad dressings, and many medications.
If your symptoms disappear during the elimination phase, the next step is a structured reintroduction. Eat a small amount of a dairy food on day one, roughly double that amount on day two, and a larger portion on day three. If symptoms return during reintroduction, that’s a strong signal you’re lactose intolerant. The key is being strict during elimination (no exceptions) and methodical during reintroduction. Eating a pizza on day one and declaring the test complete won’t give you useful information.
This method won’t give you a clinical diagnosis, but it’s practical and free. Many doctors actually start here, using your symptom response to dairy avoidance as initial evidence before ordering a formal test.
Genetic Testing
A genetic test can tell you whether you carry the gene variant associated with primary lactose intolerance, which is the most common type. The variant in question is called LCT-13910C>T, and it influences whether your body continues producing the enzyme that digests lactose into adulthood.
People with two copies of the C version (the CC genotype) are genetically predisposed to lose their ability to digest lactose over time. Those with two copies of the T version typically maintain lactose digestion throughout life. Having one of each (the CT genotype) falls somewhere in between, with a partial reduction in enzyme production that may or may not cause noticeable symptoms.
Genetic testing is done through a blood draw or cheek swab and can be useful when breath test results are unclear or when you want to understand whether your intolerance is genetic rather than caused by something else, like gut damage from an infection or celiac disease. The limitation is that carrying the gene variant doesn’t guarantee you’ll have symptoms. Some people with the CC genotype tolerate small amounts of dairy without problems.
The Blood Glucose Test
This older method works on a simple principle: when your body properly digests lactose, it breaks down into glucose and galactose, causing your blood sugar to rise. If your blood sugar doesn’t rise after drinking a lactose solution, it means the lactose passed through undigested. You’ll have blood drawn at intervals over a two-hour period after drinking the lactose load.
The blood glucose test has largely been replaced by the hydrogen breath test because it’s less accurate and more invasive (multiple blood draws versus breathing into a tube). It can also be unreliable in people with diabetes or other conditions that affect blood sugar regulation. Some clinics still offer it, but if you have a choice, the breath test is the better option.
Testing in Infants and Young Children
Breath tests don’t work well for babies and very young children, so doctors use a stool acidity test instead. When lactose goes undigested, bacteria in the colon convert it into lactic acid and other acids that lower the pH of the stool. A stool pH below 5.5 indicates carbohydrate malabsorption, even without other markers present. This test is simple and noninvasive, requiring only a stool sample.
Lactose intolerance in infants is rare. Babies are biologically designed to digest breast milk, which is rich in lactose. When a baby does show signs of trouble with lactose, it’s often temporary and caused by a gut infection or another underlying condition rather than a permanent genetic trait.
What Your Results Actually Mean
A positive test tells you that your body doesn’t fully break down lactose, but it doesn’t tell you how much dairy you can handle in practice. Lactose intolerance exists on a spectrum. Some people can drink a small glass of milk without issues but react to a large latte. Others can’t tolerate even the trace amounts found in processed foods.
It’s also worth knowing that lactose malabsorption and lactose intolerance aren’t exactly the same thing. Malabsorption means the lactose isn’t being digested. Intolerance means the malabsorption is causing symptoms. Some people show clear malabsorption on a breath test but rarely notice discomfort in daily life, because their gut bacteria handle the undigested lactose without producing much gas.
One practical concern flagged in genetic research: people with the CC genotype (genetic lactose intolerance) tend to have lower vitamin D levels, largely because they avoid dairy products that are commonly fortified with vitamin D. If you’re diagnosed and cutting back on dairy, it’s worth paying attention to your calcium and vitamin D intake from other sources.