FOOD ALLERGY TESTS
How to prepare for allergy testing? Antihistamine and antidepressant drugs should be stopped 3-5 days before testing, but only with doctor’s approval.
Skin Prick Test
A skin prick test can confirm allergies to certain foods. A drop of a dissolved food sample is placed on the forearm, and pricked with a needle and thus pushed beneath the skin; a red bump, appearing in 10-15 minutes (in delayed reaction in several hours) speaks for allergy (1). False positive results are common but false negative results are rare. The size of the skin reaction does not reflects the severity of allergy. Rarely, an anaphylactic reaction (a life threatening immune response) may occur during the test.
Patch Skin Test
A patch skin test is used when delayed allergic reaction is expected. Patches with different food samples are placed on the back for 48-72 hours and resulting skin reaction is observed (2).
RAST, a Radio-Allergo-Sorbent Test, measures the amount of IgE antibodies in the blood serum. RAST is used in small children with undeveloped immune system, in pregnancy, in expected strong allergic reaction, or when antihistamine drugs can not be safely stopped before the test. In a laboratory, suspected radio-labeled allergen is added into a blood sample, and if specific IgE antibodies are present, antibody-allergen complexes are formed and can be measured by the radio-detector. RAST is less accurate than skin prick test. Cross reactivity with some respiratory allergens is possible and IgE from outgrown allergies may still be present. In some allergies, IgE are not elevated enough to be detected. Amount of specific IgE antibodies does not necessary correlates with severity of symptoms. It may take few days to get results of a RAST test.
FOOD INTOLERANCE TESTS
Hydrogen Breath Test with Lactose (in Lactose Intolerance)
How to prepare? Do not eat for 12 hours (overnight) before the test, do not take aspirin or smoke few days before the test. The test is not appropriate for persons on antibiotic treatment and small children.
Procedure. A fluid with radio-labeled lactose is drunk. In lactase deficiency (lactose intolerance), due to lack of enzyme lactase, lactose will not be digested and absorbed in the small intestine, so it will reach the colon, where normal colonic bacteria will break it down and produce hydrogen. This hydrogen is detected when patients exhales into a testing machine. From the amount of exhaled hydrogen, an amount of non-digested lactose and hence an extent of lactase deficiency is evaluated. The test takes about two hours.
Lactose Intolerance Test (in Lactose Intolerance)
Lactose intolerance test is done when a breath test with lactose is not available (but not in diabetics and small children). You will drink a solution with lactose. If you have lactase deficiency, lactose will be only partly broken down to glucose and galactose so only a small amount of both will appear in the blood after their absorption. Your blood will be drawn and glucose levels checked every 30 minutes for two hours.
Stool Acidity Test (in Lactose Intolerance in Small Children)
Stool acidity test is used to confirm lactose intolerance in small children. A small amount of lactose is given to a child. If a child has lactose intolerance, unabsorbed lactose will reach its colon where normal colonic bacteria will break it down to lactic acid and lower pH of the stool. From the pH of the stool, an extent of lactose intolerance can be evaluated.
Hydrogen Breath Tests with Fructose (in Fructose Malabsorption)
Breath test with fructose uses the same principle as hydrogen test with lactose (see above), and it can reveal fructose malabsorption.
Elimination diet test may be tried in the hospital, when allergy tests are negative, but food intolerance is still suspected. Many or even all regular foods may be eliminated at once and an elemental diet, which is generally well tolerated, given for 1-4 weeks, until diarrhea improves. Certain food samples or placebo are then added one by one by a third party dietitian without patient or his/her doctor knowing what the sample contains (double blind placebo controlled food challenge). Irritant food often causes diarrhea within minutes or few hours after the testing meal. If there is no diarrhea, the next day a challenge with a new food sample is performed. The test bases on the increased irritant effect of a previously eliminated food (3). It may take a month or more to find a problematic food. If food allergy is still suspected, despite negative skin prick and blood tests, an oral challenge test may be done. A sample of suspected food is placed over the lower lip, and, if no reaction, into a mouth, and symptoms of allergy are monitored in the next 20 minutes. In a drug challenge test, diluted solution of a suspected drug is injected into a vein in subsequent increasing concentrations, and symptoms are observed (4).
Some simple elimination diet tests may be done at home:
- Lactose-free diet should stop diarrhea in 1-2 days in a person with lactose intolerance.
- Gluten-free diet should lessen diarrhea in some days in celiac disease; full recovery may last for several months to two years, though.
- Low-fructose diet for 4 days leads to dramatic improvement of symptoms in fructose malabsorption or hereditary fructose intolerance (HFI).
- Low-FODMAP diet with low glucose, fructose, lactose, oligosaccharides (fructans, inulin) and polyols (sorbitol) can help in small intestinal bacterial overgrowth (SIBO) , diverticulitis or other causes of diarrhea and bloating.
- Low-fat diet should help in diarrhea from fat malabsorption (due to liver, pancreatic, bile duct, celiac or Crohn’s disease) in 1-2 days.
- Low-fiber diet with limiting raw vegetables, fruits and cereals may help in IBS, hyperthyroidism and in other cases of mild diarrhea and bloating.
- Skin prick test (allergyuk.org)
- Skin patch test (allergyuk.org)
- Elimination diet (allergyclinic.co.uk)
- Drug challenge test (health.qld.gov.au)
Article reviewed by Dr. Greg. Last updated on April 12, 2011