How lactose intolerance is diagnosed: the tests, and what each one proves

Last updated 12 August 2026

There are four common ways to answer ”am I lactose intolerant?”, and they answer subtly different questions. One of them measures gas, one measures blood sugar, one reads your DNA, and one is just you, paying attention. Knowing which is which stops you buying the wrong answer.

First, two different questions

Doctors separate them, and it clears up most of the confusion:

Lactose malabsorption — your small intestine doesn’t break down lactose, so it reaches your colon. A lab can measure this.
Lactose intolerance — you get symptoms when it does. This is a matter of how you feel, and no machine measures it for you.

Plenty of people malabsorb lactose and feel fine. That’s why a positive test result isn’t a sentence, and why the only truly useful question is how much can I have before I’m uncomfortable — a threshold, not a yes or no.

The hydrogen breath test

The usual first-line test. You fast, drink a measured dose of lactose in water, and blow into a device every 15–30 minutes for two to three hours. If lactose reaches your colon, the bacteria there ferment it and produce hydrogen, which crosses into your blood and comes out in your breath. A rise above a threshold (commonly 20 ppm over baseline) counts as malabsorption.

What it’s good at: it’s non-invasive, widely available, and because symptoms are usually recorded alongside the readings, it captures both halves of the picture.

Limits worth knowing: a minority of people carry gut bacteria that produce methane rather than hydrogen, and can test falsely negative unless methane is measured too. Recent antibiotics, smoking, and exercise during the test all skew results.

The lactose tolerance test (blood glucose)

Older, and less pleasant. Same lactose drink, but instead of your breath they take blood over two hours. If you digest lactose, it splits into glucose and galactose and your blood glucose rises. A flat curve means it isn’t being digested.

Mostly superseded by the breath test, and it needs several needles rather than a tube.

The genetic test

A cheek swab or blood sample looks at variants near the LCT gene (in the MCM6 region) that control whether lactase production keeps going into adulthood. It tells you whether you’re likely to be a “lactase persister”.

What it can’t do is tell you what to eat. Genotype predicts your enzyme status, not your symptom threshold — and the threshold is the part you actually live with, because it depends on your colonic bacteria, on whether you eat dairy with meals, and on how regularly you have any. Two people with the same genotype can have very different everyday experiences of milk.

This is why we don't ask you to test before starting. No test result tells you today's safe dose. The only thing that does is a small dose and an honest note about how it went — which is the loop Lactico automates. What a test does usefully do is rule other things out, which matters if your symptoms are unusual.

Elimination and reintroduction

Cut lactose for two to four weeks, then bring it back deliberately and watch what happens. Free, and it answers the practical question directly.

Two cautions. Do it as a structured reintroduction rather than an all-or-nothing dairy binge, or you’ll learn only that a large dose hurts. And don’t run a long elimination diet if there’s any chance you have coeliac disease — removing gluten or dairy before testing can obscure a real diagnosis. Get the diagnostic work done first.

Do you need a test at all?

Often, no. If your symptoms are the ordinary gut ones, they follow dairy within a couple of hours, and they scale with how much you had, that pattern is fairly characteristic, and a supervised reintroduction will tell you more about daily life than a lab number will.

You should see a clinician rather than self-diagnose if:

your skin or breathing is ever involved — that’s the allergy question, and it’s the one that’s dangerous to get wrong;
there’s blood in your stool, weight loss you can’t explain, fever, vomiting, or diarrhoea that wakes you at night;
symptoms started suddenly in adulthood without an obvious trigger — that can point to a secondary cause worth treating;
you have a family history of coeliac disease or inflammatory bowel disease.

After the answer

If it comes back as ordinary lactose malabsorption, the practical question becomes how much you can comfortably have — and that number isn’t fixed. Regular, gradually increasing exposure usually raises it, because your colonic bacteria adapt to the work. Your lactase status stays exactly as your genes wrote it. What changes is how much you can enjoy before it costs you anything.

Sources: NIDDK, Diagnosis of Lactose Intolerance; Deng et al., Nutrients 2015; StatPearls, Lactose Intolerance; MedlinePlus on lactase persistence genotyping (LCT/MCM6). General information, not a diagnosis.