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Since lactose intolerance is the normal state for most adults, and not a disease condition, diagnosis is not necessarily required. However, if it is necessary to confirm lactose intolerance (for example, in a patient of European ancestry in whom a genetic tolerance for lactose might normally be expected), a lactose tolerance test, a hydrogen breath test, or a stool acidity test is required for a clinical diagnosis.
Milk AllergyThe differential diagnosis must distinguish lactose intolerance from milk allergy, which is an abnormal immune response (usually) to milk proteins.
Since the majority of northern Europeans and some Mediterranean Europeans have the genetic mutation rendering them lactose-tolerant, lactose intolerance is widely regarded as a medical condition in Europe and North America. A fair proportion of patients with symptoms of irritable bowel syndrome actually have lactose intolerance without knowing it.
Test for Lactose IntoleranceA simple test can clarify the issue: after an overnight fast, 50 grams of lactose (in a solution with water) are swallowed. If the lactose cannot be digested, enteric bacteria metabolize it and produce hydrogen. This can be detected in the air the patient exhales. The test takes about 2 to 3 hours. A medical condition with similar symptoms is fructose malabsorption.
Measuring the blood glucose level every 10–15 minutes after ingestion will show a "flat curve" in individuals with lactose malabsorption, while the lactase persistent will have a significant "top", with an elevation of typically 50–100% within 1–2 hours.
A lactose tolerance test, a hydrogen breath test, or a stool acidity test provides a clinical confirmation of lactose intolerance. A definitive diagnosis for research purposes can be obtained by analysis of an intestinal biopsy for lactase activity.
An informal test of lactose digestion can be done on someone by having the person follow these steps:
• Drink two cups of milk on an empty stomach and watching for signs of intestinal discomfort over the next several hours.
• The following day, eat 60 g (two ounces) of hard cheese or drink two cups of lactose-free milk.
If symptoms are experienced only on the first day, the person being tested may be lactose-intolerant. If symptoms are experienced on both days, the person may have an allergy to dairy products, which is unrelated to lactose intolerance.
Some individuals are able to self-diagnose without intentionally testing themselves, simply by realizing in retrospect that their symptoms always correspond to prior lactose consumption. This, however, is the least reliable of the methods herein described, as people's memories are imperfect. In addition, it is harder to isolate one definite cause without a formal test.
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