Food Allergy Diagnosis
History is the most important step in the diagnosis of food allergy. Testing by itself is not reliable, studies have shown a 40-50% false positive rate (positive test... but no reaction with exposure). A test is not positive until there is a positive test plus either a strong history ( ie- ate food and quick onset of reaction) or an oral challenge. There are a handful of studies that have show if the skin test reaction reaches a certain size or if the blood level of IgE reaches a certain threshold, then the positive predictive value (PPV) significantly goes up (in the case of peanut the PPV can reach near 100%).
PPV= proportion of positive test results that are true positives.
prick (percutaneous skin testing)- SPT
Used to identify the food(s) that may be causing an IgE mediated food allergy.
With an SPT a needle is used to place a tiny amount of food extract (or fresh food) just below the surface of the skin on your lower arm or back.
- SPTs are virtually pain free
SPTs can identify foods against which you have made IgE antibodies.
The results of an SPT usually appear within 15-20 minutes.
A positive SPT result is a raised bump with redness around it, called a wheal and flare. This occurs when a food allergen reacts with its IgE antibody.
A positive SPT result does not mean that you are allergic to the food. A positive result shows that you have made IgE antibodies to the food.
Occasionally, even when the food allergy involves IgE, negative SPT results do occur. (false negative)
Allergen-specific IgE in the blood
- Measuring IgE levels can be useful in identifying possible food allergens.
- Serum testing can be especially useful if SPTs cannot be done. For example, SPTs cannot be done when you have extensive eczema or when you need to take antihistamines.
- The results of IgE testing and SPTs do not always match up.
- Undetectable IgE levels occasionally occur in people with food allergy involving IgE.
food challenge test
Note: Because an oral food challenge test always carries a risk, it must be performed by a healthcare professional trained in how to conduct this test and at a medical facility that has appropriate medicines and devices to treat potential severe allergic reactions.
An oral food challenge test includes the following steps:
- You are given doses of various foods, some of which are suspected of triggering an allergic reaction.
- Initially, the dose of food is very small, but the amount is gradually increased during the challenge.
- You swallow each dose.
- You are watched to see whether a reaction occurs. The first sign of the reaction , the test is stopped (this is a test for +/- allergy, not to test severity of reactions).
- If an oral food challenge test results in no symptoms, then food allergy can be ruled out.
- If the challenge results in symptoms and these symptoms are consistent with your medical history and laboratory tests, then a diagnosis of food allergy is confirmed.
Predictor of severity of reactions
Allergy testing is not a gauge of severity of reaction.
In general the larger the skin reaction and the higher the specific IgE is the more likely it is a true positive (but false positive results always exist). Also, the larger the skin reaction and higher the specific IgE the higher likelihood of more severe reaction.
The tests are just a test for the presence of IgE antibodies produced by your body for specific proteins (peanut, casein, whey, egg etc.). How your body responds to the presence of these antibodies is the root of the reaction severity.
Example: A person can have a very large skin test reaction (20mm wheal) and specific IgE >100 and only have a few hives or even no reaction with exposure. Another person can have a skin teat with a small skin reaction (4mm wheal) and blood testing specific IgE 5, upon exposure to the protein they have an anaphylactic reaction.
Tests that should not be done for the diagnosis of food allergy. (either dangerous or a waste of money)
Intradermal test: An
intradermal test is performed by injecting a small amount of
the skin. There is no clinical evidence to show that intradermal testing is more useful than an SPT to diagnose food allergy. The false positive reactions are very very high. You also are much more likely to have a severe adverse reaction to an intradermal test, compared with an SPT.
Total serum IgE: There is not enough clinical evidence to show that measuring total serum IgE levels is sensitive or specific enough to diagnose food allergy.
IgG testing, ALCAT, Cytotoxic testing, Applied kinesiology, NAET testing- none of these testing modalities are supported by and scientific studies. Many have studies that totally refute any utility and are not valid. Not worth our time (yours and mine) to further go into detail.