Yes, there are several reliable tests for lactose intolerance. The most common is the hydrogen breath test, a noninvasive procedure that takes two to three hours and can be done at a doctor’s office or gastroenterology clinic. Other options include a blood glucose test, genetic testing, stool acidity testing for young children, and a simple at-home elimination approach.
The Hydrogen Breath Test
The hydrogen breath test is the most widely used diagnostic tool for lactose intolerance, and the one your doctor is most likely to order. It works on a straightforward principle: if your body can’t break down lactose, bacteria in your large intestine will ferment it instead, producing hydrogen gas. That gas gets absorbed into your bloodstream, travels to your lungs, and shows up in your breath.
Here’s what the test looks like. You’ll start by breathing into a balloon-like container or bag attached to a measuring device, which records your baseline hydrogen level. Then you’ll drink a flavored liquid containing a measured dose of lactose. Over the next two to three hours, you’ll breathe into the container again every 15 to 30 minutes. Each sample is measured for hydrogen, and you’ll be asked to note any digestive symptoms along the way, such as bloating, cramping, gas, or diarrhea.
If your hydrogen levels rise significantly above your baseline reading, it indicates that undigested lactose reached your colon and was fermented by bacteria. The test is painless, though if you are lactose intolerant, the lactose drink will likely trigger your usual symptoms during the procedure. Those symptoms are temporary and resolve as the lactose passes through your system.
How to Prepare for a Breath Test
You’ll need to fast before the test, typically overnight or for at least 8 to 12 hours. Your provider may also ask you to avoid certain medications, particularly antibiotics, in the weeks leading up to the test, since antibiotics can alter your gut bacteria and skew the results. Smoking before the test can also interfere with readings.
On the day of the test, avoid eating or drinking anything other than water until the procedure is complete. The goal is to make sure any hydrogen detected comes from the lactose solution, not from food you ate earlier.
The Blood Glucose Test
A less common alternative is the lactose tolerance blood test. Instead of measuring hydrogen in your breath, it tracks how your blood sugar responds after you drink a lactose solution. Normally, your small intestine breaks lactose into two simpler sugars, glucose and galactose, which then enter your bloodstream. If you’re producing enough of the enzyme that digests lactose, your blood glucose will rise noticeably.
Blood samples are drawn before and at intervals after you drink the lactose solution. The test is considered normal if your blood glucose rises more than 30 mg/dL within two hours. A rise of less than 20 mg/dL is considered abnormal, suggesting your body isn’t breaking down the lactose. A rise between 20 and 30 mg/dL falls into an inconclusive range, meaning further testing may be needed. This test is used less frequently than the breath test because it requires multiple blood draws and can be influenced by other factors that affect blood sugar, like diabetes.
Genetic Testing
A genetic test can identify whether you carry the DNA variants associated with lifelong lactose digestion. Most humans are actually born with the ability to digest lactose, but the majority of the world’s population gradually loses that ability after childhood. A small number of genetic variants, located in a regulatory region near the gene that controls lactase production, allow some people to keep producing the enzyme into adulthood.
A genetic test, usually done with a blood sample or cheek swab, checks for those variants. If you don’t carry any of them, you’re genetically predisposed to becoming lactose intolerant. The limitation is that genetics can tell you about your predisposition but not how well you’re actually digesting lactose right now. Someone with the “intolerant” genotype might still tolerate small amounts of dairy without symptoms. For that reason, genetic testing is sometimes used alongside a breath test rather than as a standalone diagnosis.
Testing in Infants and Young Children
The breath test requires a patient to follow instructions and provide samples over a few hours, which makes it impractical for babies and very young children. Pediatricians instead use stool tests. One version checks the acidity (pH) of a child’s stool. When lactose isn’t properly digested, it ferments in the gut and produces acids, making the stool more acidic than normal. A second stool test looks for the presence of glucose, which signals undigested lactose passing through the digestive system.
These tests are less precise than the breath or blood tests used in older children and adults, but they provide useful evidence when combined with a child’s symptom pattern.
Small Bowel Biopsy
The most direct way to confirm lactose intolerance is to measure the actual enzyme level in the lining of the small intestine. This requires a biopsy, where a tiny tissue sample is taken during an endoscopy and analyzed in a lab. A very low lactase reading confirms the diagnosis.
In practice, this is rarely done solely to diagnose lactose intolerance because the breath test is so much simpler. A biopsy is more likely to happen if you’re already having an endoscopy for another reason, such as investigating celiac disease or another condition that damages the intestinal lining and can cause secondary lactose intolerance.
The Elimination Diet Approach
Many people never get a formal test at all. Instead, they try removing all lactose from their diet for two to three weeks and track whether their symptoms improve. If bloating, gas, cramping, and diarrhea clear up during the elimination period and return when dairy is reintroduced, that’s strong practical evidence of lactose intolerance.
This approach works well if your symptoms are clear-cut, but it has limitations. Other conditions, including irritable bowel syndrome and sensitivity to milk proteins (which is different from lactose intolerance), can cause similar symptoms. If you try eliminating lactose and your results are ambiguous, or if symptoms persist, a formal breath test can give you a more definitive answer.
One useful strategy if you suspect intolerance but aren’t ready for a clinical test: try a small amount of a dairy product and wait to see how you feel before consuming more. This can help you gauge your personal threshold, since many people with lactose intolerance can still handle small servings of dairy without symptoms.