Lactose Tolerance Test

Evaluate lactose intolerance, malabsorption syndromes. May be abnormal with Crohn's disease, small bowel resections, jejunitis, Giardia lamblia infestation, Whipple disease, and cystic fibrosis.

Lactose is a disaccharide digested by lactase. It yields glucose and galactose. The latter is converted to glucose by the liver after its absorption. Glucose is measured and it is the increase or lack of increase over the fasting specimen that is used for interpretation. A normal response will show a glucose increase >30 mg/dL over the baseline sample. Those with lactase deficiency will demonstrate bloating, cramps, and diarrhea and will show a glucose increase <20 mg/dL. Diabetic patients may have abnormal lactose tolerance curves due to abnormal carbohydrate metabolism and not necessarily due to lactose intolerance. Since 25% of normal individuals have flat glucose tolerance tests, it has been suggested that patients with flat lactose tolerance tests should also have a glucose tolerance test. Ethanol can prevent conversion of galactose to glucose by the liver; thus, blood or urine galactose can be measured.

Test may produce diarrhea and cramps.

Heyman MB; Committee on Nutrition. Lactose intolerance in infants, children, and adolescents. Pediatrics. 2006 Sep; 118(3):1279-1286.

Wu A. Tietz Guide to Laboratory Tests. 4th ed. Saunders/Elsevier; 2006.

Patient should fast for eight hours before testing. No smoking or gum chewing allowed during test. Patient is encouraged to drink a moderate amount, one to two glasses of water during test. Patient should remain seated or in bed during test.