Tests
1st tests to order
trial of dietary lactose elimination
Test
Diagnostic test of choice.
Result
resolution of symptoms once lactose-containing dairy and nondairy products are eliminated from diet; resumption of symptoms when reintroduced
CBC
Test
Normal results do not differentiate lactase deficiency. However, if anemia is present, it may point toward the underlying cause of secondary disease (e.g., celiac disease, eosinophilic enteritis). If white cell count is elevated, it may point toward infective causes of secondary disease.
Result
primary lactase deficiency: normal; secondary lactase deficiency: may be elevated white blood cell count, may be reduced hemoglobin
Tests to consider
lactose hydrogen breath test
Test
Used if dietary elimination and subsequent challenge are inconclusive.[2]
Values between 10 and 19 parts per million may be indeterminate unless accompanied by symptoms.
Noninvasive, easy-to-perform, with high sensitivity and specificity.[27]
Accurate results require proper equipment and preparation. Certified medical products for collection should be used. Ideally the patient should be fasting for at least 8 hours. Smoking and exercise may induce hyperventilation, and should be avoided. Guidelines recommend delaying breath testing until 4 weeks after completion of antibiotic therapy and 2 weeks after colonic cleansing.[27][28]
Result
breath hydrogen ≥20 parts per million after lactose load and intolerance symptoms
stool culture
Test
Used rarely, in patients with short histories of diarrheal illness.
Positive in patients with infection (causing secondary disease); negative in primary lactose intolerance.
Result
positive or negative
fecal pH
Test
Useful test in infants.
Reduced in hypolactasia, due to the formation of volatile fatty acids as a result of carbohydrate malabsorption.
Lower sensitivity and specificity than the lactose hydrogen breath test, and does not differentiate lactose from other carbohydrate (fructose, glucose, and galactose) malabsorption.
Lower in infants than in older children.
Result
reduced fecal pH
fecal reducing substance/sugar
Test
Fresh stool samples required and assays should be performed immediately.
Presence of reducing substances/sugars in feces indicates carbohydrate malabsorption.
Does not differentiate lactose from other carbohydrate (fructose, glucose, and galactose) malabsorption.
Even less sensitive than fecal pH.
Result
positive for reducing substances/sugars
lactose tolerance test
Test
Serum glucose is measured after fasting. Lactose is then administered (50 g for adults, 2 g/kg for children). Serum glucose is then remeasured at 0, 60, and 120 minutes.
Time-consuming and produces quite high rate of false-positives and false-negatives.[2]
Has largely been replaced by lactose hydrogen breath test.
Result
elevated serum glucose after lactose load <20 mg/dL (1.1 mmol/L) and/or symptoms produced
small bowel biopsy
Test
Performed in patients with persistent symptoms and positive celiac serology, patients with convincing histories of exposure to giardiasis (e.g., well-water, camping) or bacterial overgrowth (e.g., prior surgery), or when lactose hydrogen breath test is not available.
Invasive, less sensitive than lactose hydrogen breath test.
Samples taken during endoscopy.
May be normal if mucosal abnormality is focal or patchy.[32]
May also show underlying cause in case of secondary lactase deficiency.
Result
normal or reduced intestinal lactase (and/or other disaccharidases)
Emerging tests
genotyping
Test
Almost all symptomatic patients with CC_13910/GG_22018 genotype are lactase deficient and lactose intolerant; 82% of CT_13910/GA_22018 patients and 68% of TT_13910/AA_22018 patients are lactose intolerant as well.
Higher sensitivity and specificity.[31][35]
Used in Germany and the Nordic countries. Not yet widely available in clinical practice elsewhere; continues to be evaluated worldwide.
Result
CC_13910/GG_22018, CT_13910/GA_22018 and/or TT_13910/AA_22018 genotype
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