Diagnosis and Testing
What is Allergy?
When the term allergy was first introduced in 1906, it meant an adverse reaction to a food or other substance not typically regarded as harmful or bothersome.
For most people this is still what allergy means, although doctors use the word rather differently and this can be both misleading and confusing. Doctors use the word allergy to mean an adverse reaction of the immune system to a substance not recognised as harmful by most people’s immune systems.
True allergies (for example allergies to pollens, dust mites, fish, shellfish or nuts) are typically associated with the formation of antibodies. Some people (doctors refer to these people as atopic) have an inherited tendency to this type of allergy and they tend also to be prone to asthma, eczema and hay fever; a condition known as atopy. In certain circumstances, and especially during the first few years of life, atopic people may develop immunoglobulin E (IgE) antibodies when exposed to an allergy-inducing protein in a process called sensitisation. When sensitisation has occurred, the allergy-inducing protein is referred to as an allergen and the resulting antibody (also a protein) as allergen specific IgE.
What is intolerance?
Whilst doctors use the term allergy when referring to an adverse reaction that involves the immune system, the term intolerance is preferred when an adverse reaction shows no evidence of immune system involvement. The scientific term for an intolerance is non-allergic hypersensitivity.
Food intolerance can occur when the body fails to produce a sufficient quantity of a particular enzyme needed to break down a food and aid digestion. For example if a person suffers wind and bloating every time they consume milk or milk products they may be suffering from lactose intolerance, a condition caused by lack of the lactose-digesting enzyme lactase.
If a person suffers nervous system symptoms because of an amount of caffeine in a mug of strong coffee that would be tolerated by most people, this person would be suffering from a drug-like or pharmacological food intolerance. These can occur either because of an intolerance of chemicals naturally present in foods (such as theobromine in chocolate or tyramine in aged cheeses) or an intolerance of food additives such as sulphites or benzoates.
Whilst enzymatic and pharmacological food reactions only affect some people, toxic food reactions affect 100% of the population provided that a sufficient amount of the food is ingested. A good example is the false food allergy type of reaction that can occur when sufficient histamine accumulates in the flesh of spoilt tuna (scombroid reaction).
None of the above examples of food intolerance involve the immune system and, for this reason, none can result in life-threatening allergy or anaphylaxis.
How Do You Know If You Have an Allergy?
The conventional allergy tests used by doctors depend on the presence of allergen-specific IgE antibodies. The two most used are the skin prick test and the specific IgE blood test (previously called a RAST test). However, it is very important to realise that whilst allergy is unlikely when allergen-specific IgE is absent, the presence of allergen-specific IgE only indicates that sensitisation has occurred, it does not diagnose the allergy. When completely healthy and symptom-free people are tested for allergy, positive results are often found. These results are called false positives.
For this reason, reliable allergy diagnosis is dependent on an allergy focused history. A good allergy clinician can usually suspect the likely allergen(s) from the history alone, and allergy tests may not be needed. However, because a negative allergy test may point to a different allergy or a different explanation, allergy tests are very useful to confirm the diagnosis. This is especially important in the case of suspected food allergy when an inaccurate diagnosis might commit the patient to lifelong, but unnecessary, food avoidance. Allergy tests are also useful if there is any confusion as to whether symptoms are being caused by true allergy or whether some other process is involved.
This is why allergy tests need to be interpreted by a healthcare professional qualified in allergy, who will interpret the results in light of an allergy-focused history. It also explains why it is important not to test everybody for every known allergen, an approach that would inevitably lead to erroneous diagnosis.
Occasionally doctors may be faced with a situation where the allergy history points strongly in one direction whilst an allergy test points strongly in another. This is when a provocation challenge test may be useful. The test is only undertaken under specialist supervision in a hospital. The patient is exposed to tiny, but gradually increasing, amounts of the suspect item (typically a food such as peanut) until there is the tiniest hint of a rash, swelling, breathing difficulty or drop in blood pressure. This is the gold standard amongst allergy tests.
What Other Allergy Tests are Available?
Contact (or ‘patch’) testing is a conventional test that is used by skin specialists in cases of contact dermatitis. The test diagnoses a delayed, or cell-mediated (as opposed to antibody-mediated) type of allergy that mainly affects the skin.
Unconventional allergy tests that are considered to be of no value include applied kinesiology (based on measurements of muscle strength), auricular cardiac reflex (based on measurements of the pulse at the wrist), hair analysis (based on a pseudo-scientific concept called bioresonance), cytotoxic test (based on an examination of the white blood cells) and the Vega test (based on acupuncture theory and electromagnetism).
Are There Reliable Tests For Food Intolerance?
With the exception of lactose intolerance, for which a conventional test exists, no reliable forms of testing exist for the types of food intolerance described above.
However, recent scientific studies are beginning to point to a delayed type food allergy in which the immune system is involved even though allergen-specific IgE is not present. For this reason, the controversial view that certain medically unexplained symptoms might be related to a delayed form of food allergy rather than be due to an unexplained or psychosomatic mechanism may yet prove to have some scientific worth. Studies that have used food exclusion followed by blinded and placebo-controlled food challenge, have suggested that this kind of mechanism may apply in some cases of migraine, arthritis and irritable bowel syndrome.
However, with the exception of dietitian supervised food exclusion and food challenge, no validated test for this type of food allergy has so far emerged. Whilst some practitioners (and especially unconventional practitioners) use IgG (as opposed to IgE) blood tests when investigating such cases, the data is not scientifically robust and most allergy specialists consider IgG blood tests to be unhelpful.
Last reviewed: October 2013 Next review date: October 2015
Video on the Subject