Skin-prick tests
This is an indirect method of detecting true allergic reactions. It is one of a family of skin tests that use a similar approach. The three different tests in this family are known as: skin-prick tests or prick tests, puncture tests, and scratch tests.
For the skin-prick test - the technique used in Britain - a small drop of liquid containing an allergen, such as grass pollen, is placed on the arm. The doctor makes a small prick in the skin, under the drop of liquid, allowing a minuscule amount of the allergen to get into the skin. A positive reaction is recorded if a red bump develops soon afterwards. For accuracy, the bump must be compared to positive and negative controls (see below).
The puncture method is very similar to the skin-prick test but uses a slightly different technique for breaking the skin. The term prick-puncture test covers both techniques.
With the scratch method, the skin is scratched lightly, and the allergen solution is then applied over the scratch. This method gives less consistent results than prick-puncture testing.
It is important to include a negative control in the test - a skin-prick test with plain salt water (saline). This should not produce much of a bump - if it does, the skin is clearly over-reactive and the tests more difficult to assess. The doctor should also include a positive control - a skin-prick test with histamine, the substance that plays a central role in allergic reactions. This should always produce a bump. If it does not, the skin is decidedly under-reactive, and the tests are invalid.
Taking antihistamines will make the skin under-reactive, and you should stop taking them before the testing, for a period ranging from a day to several weeks - it varies depending on the particular antihistamine. Ask your doctor for specific instructions about stopping these and other drugs before testing.
Skin tends to be over-reactive to testing in people with dermatographism (see p. 52). Blood tests for specific IgE,
such as RASTs (see p. 92), are needed for anyone who has this condition. Eczema sufferers with a rash over large areas of the body may also require blood tests, if there is too little clear skin for testing.
Skin-prick tests can produce both false positives and false negatives (see box below). Some allergic diseases will give a lot of false negatives and relatively few false positives, while for others the reverse is true. The allergen itself influences the rates of misleading reactions: for example, tests for soya allergy are notoriously unreliable, whereas those for peanut are far more accurate. The age of the person being tested also makes a difference. With all these influences at work, interpreting the test responses is a real art, and the doctor’s experience counts for a lot.
All sorts of people offer skin-prick tests, including alternative practitioners. Get them done by a qualified doctor, preferably by an allergist, who will know how to make sense of the reactions.
Note that the purpose of these tests, and of blood tests for specific IgE, is to identify the allergens that are bringing on your symptoms, not to predict how strongly you will react to those allergens. The tests may give some indication of the intensity of your reaction, but they cannot be regarded as a good guide to how you will respond to the allergen in the future.
The safety record of skin-prick tests is very good. Occasionally a systemic reaction (anaphylaxis) occurs with these tests, but there are no records of any deaths. Nevertheless, if you suffer from severe asthma or have experienced anaphylactic shock in the past, it is advisable for the doctor to have adrenaline and resuscitation equipment available. Those with strong allergic reactions to latex may also react badly if they are tested with an allergen that cross-reacts with latex (e.g. cypress pollen), not just when tested with latex itself. Taking beta-Mockers (see box on p. 150) increases the risk of a life-threatening reaction for anyone in these higher-risk categories.
False positives and false negatives
Apart from challenge tests, none of the tests used for allergy works with 100% accuracy. Most give both false positives and false negatives.
A false positive means that there is a positive test but no actual reaction when the allergen is encountered (e.g. eaten or inhaled). A false negative means that there is a negative test result despite a genuine reaction (as shown by a challenge test, for example).
A test that gives relatively few false positives has good positive predictive value - in other words, if it suggests you are allergic to something, you probably are.
A test that gives relatively few false negatives has good negative predictive value. If it comes up negative, you are probably not allergic to that allergen.
Some tests for allergic reactions show good positive predictive value but poor negative predictive value, while for other tests the reverse is true.
Fresh is best
The fruit and vegetable allergens that provoke Oral Allergy Syndrome (see p. 63) are chemically unstable, so commercially produced extracts for skin-prick testing quickly lose their potency and give false-negative results. Most allergists now favour using a drop of fresh juice from the fruit or vegetable concerned.
Intradermal tests
These tests (also called ‘intracutaneous tests’) put allergen more deeply into the skin than prick-puncture tests. The skin tends to react more when penetrated to this depth, so there are more false positives. There is also a greater risk of a serious reaction which may require emergency resuscitation. Don’t undergo these tests if you are taking beta-Mockers (see box on p. 150).
Blood tests for IgE
There are blood tests that look at the total amount of IgE (the allergy antibody), which is sometimes useful in diagnosis. But more important are blood tests for specific IgE – against egg or grass pollen or latex, for example. There are different ways of measuring the IgE in the blood, the most commonly used being a radio-allergosorbent test or RAST.
Research shows that RASTs are no more accurate than skin-prick tests in confirming real-life allergic reactions. However, they are useful for patients who can’t discontinue their antihistamines without developing severe symptoms, and for those with dermatographism or very severe eczema (see p. 91).
Patch tests
These tests, used primarily for contact dermatitis, are similar to straightforward challenge tests, because the suspect substances are applied directly to the skin.
The test substances are placed on the skin – usually on your back – in small chambers. They are held in place with sticky tape, and left there for several days. Ideally, the reaction of the skin should be checked three times: after two days, again the next day, and again the day after that. It really is worth going back for all these separate visits, because the accuracy of the test increases greatly with repeated checking.
The substances chosen for testing are a standard set of antigens that most commonly cause contact dermatitis. This standard set will pick up 60-80% of all sensitivity reactions in contact dermatitis. If you have substances that you suspect may be causing symptoms, such as cosmetics, the doctor can usually test for these too.
You should not be tested while you still have a rash, as the testing will probably make the existing rash flare up, even though the test patches are applied well away from the rash.
Use of steroid creams and any light treatments (including exposure of the test area to ordinary sunlight) must stop at least a week before testing starts, or the results will not be accurate.
Interpreting patch tests requires a huge amount of skill, plus extensive knowledge of the finicky details of the different test substances. You need a dermatologist with considerable experience in this area.
False positives (see box on p. 91) can occur, especially if you react very strongly to one of the substances tested – some people develop what dermatologists call an ‘angry back’, and this generates false positives to various other substances being tested at the same time. Should you be told that you are sensitive to a great many different things, you may want to query this reading of the test. Ernest N. Charlesworth, an allergist and dermatologist at the University of Texas, describes patients who ‘develop into environmental cripples’ after being told that they are definitely sensitive to multiple antigens, on the basis of misinterpreted false-positive patch tests.
False negatives (see box on p. 91) are also possible, even with very careful testing. Should this occur, a type of challenge test known as a ROAT (Repeat Open Application Test) is possible. The suspect substance is applied to the inner fold of the elbow twice a day for a week. Get your doctor’s agreement before trying this test.
Endoscopy and biopsy
Miniaturised cameras and sophisticated fibre-optics have allowed modern doctors to do something that their predecessors could never have imagined possible – look right inside the human body. This procedure is called endoscopy, and it has a useful role in a few sensitivity reactions.
Looking inside the sinus cavities can assist in understanding exactly what is going wrong in chronic sinusitis. Inspecting the digestive tract can be valuable in several of the non-IgE immune reactions to food, such as coeliac disease (see p. 70) and eosinophilic gastroenteritis (see p. 72).
A biopsy is often carried out at the same time as endoscopy.
s involves taking a small sample from the affected area, such as
I ning of the gut, and studying it in detail under a microscope.
One purpose of a gut biopsy is to look for characteristic :goes of damage to the lining of the gut – such as the distinctive charges produced by untreated coeliac disease. A biopsy can also reveal what kind of immune cells are present. Abnormal numbers of certain immune cells, for example, eosinophils (see p 19), may suggest a particular diagnosis.
Another way of looking at what kind of immune reactions are going on, used for lung diseases, is a bronchoalveolar lavage – iterally a ‘washing out’ of the airways and lungs, allowing immune cells to be collected and studied. This diagnostic technique is lased for Heiner’s Syndrome (see p. 72).
Tests for food intolerance
The only really effective way of testing for food intolerance is an el ruination diet (see pp. 194-7). This is the gold standard. However, it is neither easy nor quick – which has led to a constant search for alternative tests.
The proposed alternatives are all indirect tests, that is to say, non-dietary. The results of the tests are used as a basis for an avoidance diet. In other words, the foods that give a positive test result are avoided.
Some of these tests use samples of hair or blood, others use pulse testing, pendulums, or muscle strength tests (’applied kinesiology’). A few of these tests do show some promise. Pulse tests, and a blood test called the ‘lymphocyte transformation test’. for example, can give a general indication of sensitivity reactions – sometimes. However, even in the most expert hands, these do not give a result that is accurate enough to be useful.
Of the other tests that are available, most have not been evaluated at all objectively.
Many of them are advertised directly to the public, and one of the problems with this approach is that the testing company starts by assuming that food is the problem. The same is usually true of ‘dietary therapists’ and others in the alternative health field offering tests of this kind.
Almost everyone who undertakes such tests is given a fairly long list of foods which have come up positive in the tests. This does not fit with the evidence from medical trials in which a group of people with irritable bowel or migraine (typical food intolerance symptoms) undertake an elimination diet. A significant proportion of them always find that they do not have food intolerance. Of the rest. many find that they react to one or two foods only. The long lists of foods produced by the commercial tests are, to put it mildly, implausible.
With tests that require a sample of blood, sending off two blood samples from the same person, under different names, is a simple way of assessing the tests’ validity. This exercise has been tried several times with different testing companies, and every time two completely different lists of foods have been sent back.
Covert studies of this kind have also shown that the tests overlook genuine reactions. In one alarming case, a woman with a true allergy to peanuts was assured by a ‘dietary therapist’ that she really could eat peanuts safely.
Many people with food intolerance will tell you that they did well after following a diet based on such tests – and they may well have done. Given that common foods such as wheat and milk are regular offenders in food intolerance, and that these foods very frequently feature on the lists of positive test results generated by commercial testing companies, quite a few people should do well. The problem is that these people may also be avoiding many other foods quite unnecessarily.
Furthermore, if people have sensitivities to some other foods that are not on the list, they will be missing out. They could enjoy a far better level of health if all the foods causing symptoms were Identified and removed from their diet.
In the end, an elimination diet is both cheaper and far more likely to give the right answers.
Testing for IgG antibodies
In diagnosing food intolerance, a few doctors offer tests for a type of antibody called IgG. This antibody is formed to any food molecules that get Into the bloodstream after a meal – and some do, even in entirely healthy people. So finding IgG antibodies to food molecules is not indicative of any disease at all. It occurs in everyone and is perfectly normal.
Nevertheless, some doctors feel that by measuring the level of IgG antibodies to foods, they can get a general idea of the permeability of the gut wall (which might possibly be true) and of particular foods that could be causing intolerance reactions (very doubtful – the tests just tell you what you eat most, and you know that already).
This test does measure something real, unlike some of the alternative tests for food intolerance. But the relevance of what it measures to the health of the individual concerned is partial and indefinite. A recent study of IgG testing for irritable bowel syndrome has confirmed this view.
In short, blood tests for IgG antibodies to food molecules seem like very poor value for money, and potentially misleading, whereas an elimination diet is a far more precise way of pinpointing food intolerances.
