Lactose Tolerance

Lactose intolerance is a common condition affecting 5 to 10% of Caucasians and the majority of African American and Asians. It is caused by a deficiency of intestinal lactase, the enzyme that hydrolyzes lactose in dairy products to glucose and galactose. Congenital lactase deficiency is extremely rare and manifests at birth as intolerance to breast milk or lactose-based formula with consequent severe diarrhea, acidosis, and malnutrition. Acquired lactase deficiency may be primary due to age-related decline in enzyme activity or secondary to other conditions that alter the intestinal mucosa such as celiac sprue, Giardia infection, Crohn’s disease, or infectious diarrhea. Most Caucasians begin a genetically programmed decrease in lactase activity after 5 years of age and develop the full extent of their deficiency between ages 8 and 15 years. In blacks and Asians, the decrease begins between 1 and 3 years of age. Lactase deficiency results in a build up of lactose in the intestine. Being a disaccharide, lactose cannot diffuse through the mucosa lining the small intestine. Increased osmotic pressure causes fluid to be secreted into the intestine, resulting in diarrhea. Bacteria ferment the lactose into carbon dioxide, hydrogen, and lactic acid, which lower the pH and cause flatulence, bloating, and abdominal cramping.

Since lactase deficiency can be inferred from the effects of milk consumption, the lactose tolerance test is not often needed for diagnosis of this condition. A trial of withdrawal of lactose containing food is advocated before performing the lactose tolerance test. The lactose tolerance test evaluates lactase deficiency by following blood glucose levels for a period of time after ingesting a standard dose of lactose. If lactose is cleaved and absorbed, an increase in blood glucose should be observed. The test is performed by having the patient ingest 50 to100 grams of lactose after an overnight fast. Blood samples are collected in gray top tubes before lactose ingestion and at 15, 30, 45, 60, and 90 minutes post-dose. Patient symptoms should be recorded throughout the study. Normally, blood glucose levels should increase 20 mg/dL above the fasting glucose level.Lactase deficient patients usually have a plasma glucose increase of <20 mg/dL and symptoms.

Lactose tolerance tests have a 20% incidence of false positive and false negative results. Up to 20% of normal individuals may have flat blood glucose responses after a lactose challenge. Some of these patients may be diabetics, who exhibit flat lactose tolerance tests due to abnormal carbohydrate metabolism. Alcohol consumption impairs the conversion of galactose to glucose. Lactose tolerance tests are abnormal in patients with other intestinal diseases such as Crohn’s disease, sprue, Whipple’s disease, and cystic fibrosis.

Reference value is a plasma glucose rise of >20 mg/dL above the fasting value.

Specimen requirement is 6 grey top tubes of blood drawn at the times specified above.

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