Symptoms including abdominal distention, flatulence, abdominal cramps and ultimately diarrhea are independent of the cause of lactose intolerance and are directly related to quantity of ingested lactose. These symptoms are not necessarily correlated with the degree of intestinal lactase deficiency. Malabsorbed lactose generates an osmotic load that draws fluid and electrolytes into intestinal lumen, leading to loose stools. As little as 12gm of lactose (amount of lactose in 8 oz glass of milk) may be sufficient to cause symptoms in children with chronic abdominal pain. (34)

In colon, bacteria metabolize unabsorbed lactose producing volatile short chain fatty acids and gases (methane, carbon dioxide and hydrogen) leading to flatulence. Fatty acids lower faecal pH making stool pH a non specific but helpful marker for lactose malabsorption. When sufficient intestinal gas in produced, intestinal distention stimulates intestinal nervous system leading to abdominal cramps.

Initial studies using lactose hydrogen breath tests documented malabsorption in up to 40% children and adolescents presenting with abdominal pain. (35) Prevalence of abdominal symptoms related to lactose intolerance documented by hydrogen breath tests is variable and ranges from 2% in Finnish children to 24% in southern US children. (36,37)

A good clinical history often reveals a relation between lactose ingestion and symptoms. When lactose intolerance is suspected lactose free diet can be tried (Table 2 & 3). (38) All sources of lactose should be eliminated requiring reading of food labels to identify "hidden" sources of lactose. Generally a 2 week trial of strict lactose free diet with resolution of symptoms and subsequent reintroduction of dairy foods with recurrence of symptoms can be diagnostic. In more subtle cases, hydrogen breath test is least invasive and most helpful test to diagnose lactose malabsorption. The test has been shown to be more reliable than history because some patients who think they are lactose intolerant when they prove not to be and vice versa. (39.40) The test is performed by administration of standardized amount of lactose 2gm/kg to a maximum of 25gm (equivalent to amount of lactose in two 8 oz glasses of milk) after fasting overnight and then measuring amount of hydrogen in expired air over 2-3 hr period. An increase (>20ppM) in the hydrogen expired after approximately 60 minutes is consistent with lactose malabsorption. Recent use of antimicrobial agents, lack of hydrogen producing bacteria (10-15% of population), ingestion of high fibre diet before test, small intestinal bacterial overgrowth or intestinal motility disorders produce false negative or false positive results. A pediatric gastroenterologist should be consulted to interpret results of this test.

Other conventional lactose tolerance test is not sensitive enough to determine if a subject is malabsorbing some lactose. Lactose intolerance was diagnosed by onset of symptoms and/or positive test result after ingestion of standard lactose dose (2gm/kg body weight or 50g/m² of BSA; maximum 50gm in a 20% water solution). If maximum increase in blood glucose concentration was less than 26mg/dl after test dose, lactose malabsorption is diagnosed. Test is often falsely positive because of lack of increase of blood glucose concentration attributable to normal insulin response to the carbohydrate load. Due to high rate of false positive and negative results, currently this test has been replaced by hydrogen breath test.

If secondary lactose intolerance is suspected, stool examination for parasites such as giardia lamblia and cryptosporidia species, blood tests for celiac disease (total immunoglobulin A concentration and anti tissue transglutaminase antibody) (38,39) or immunodeficiency (quantitative immunoglobulins) should be undertaken. Intestinal biopsy may be needed to uncover any gastrointestinal mucosal problem. Status of brush border disaccharidases like lactase, sucrase, and maltase and isomaltase can be directly measured. However intestinal lactase concentration does not seem to correlate well with symptoms of lactose intolerance. (41)

Eventually newer tests like {¹³c} lactose breath test are being considered to augment accuracy of breath hydrogen test. (42,43) Newer tests may yield additional information pertaining to prevalence and significance of lactose intolerance. (44)

In infants faecal pH can be tested which decreases as a result of formation of volatile fatty acids from malabsorbed carbohydrates. Faecal pH is normally lower (5.0-5.5) in infants compared to older children and adolescents because of physiological overload of lactose in their diets which favors growth of lactobacillus species in the colon. Faecal reducing substances can be measured. Reducing sugars include lactose, glucose, fructose and galactose but not sucrose. Stool pH is move sensitive test for carbohydrate malabsorption because some patients may only malabsorb enough carbohydrates such as lactose to lower faecal pH but not increase excretion of carbohydrate in the stools.