Accurate Diagnosis
The simplest and most certain test for any sensitivity reaction is to expose the person concerned to the substance under suspicion and see what happens. This is known as a
challenge test. With true allergies, challenge tests are powerful tools, but they are also alarmingly close to reality. The risk of provoking a severe reaction requires a very
cautious approach.
By comparison, an indirect test – a roundabout way of seeing how the body responds, such as the skin-prick test (see p. 91) – has the advantage of rarely producing dangerous
reactions. The downside is that indirect tests can be misleading, precisely because they are not like the real-life situation. No indirect test is perfect – there are always
false positives and false negatives (see box on p. 91).
Challenge tests
If you undergo a challenge test with food or an airborne allergen, you will also be given dummy challenges with an innocuous substance which is indistinguishable from the item
being tested. Neither you, nor the tester who is scoring the reaction, should know which is which. This is called a double-blind trial because, to eliminate all possible bias,
both of you are in the dark. (The full name is a ‘double-blind placebo-controlled trial’ – the dummy challenge is also called a ‘placebo challenge’ or ‘control challenge’.)
The double-blind trial is a standard medical procedure and does not imply that the doctors think you are faking symptoms. Psychological forces are powerful things, and just
thinking that you might react to a test can be enough to produce a reaction – the process that generates the symptoms is largely unconscious.
Food challenge
A food challenge – eating the food that is under suspicion – is a key test for food intolerance (see p. 197). It is sometimes used for food allergy and other forms of food
sensitivity too, as a follow-up to skin tests. Some allergists use a food challenge only if the skin test is at odds with actual events reported by the patient. Other allergists
use food challenge more readily, to confirm skin-test results, and to assess the severity of the reaction.
Extreme caution must be exercised with immediate food allergy, because of the considerable risks involved. The test must be done under medical supervision with resuscitation
equipment to hand. A challenge test should never be done for true food allergy without some careful preliminary tests on the face and the lips (see box on p. 23). Even if these
tests produce no reaction, only tiny amounts of the food should be eaten to begin with.
Bronchial challenge
This type of test involves inhalation of an airborne allergen – such as pollen – suspected of causing asthma. Bronchial challenge carries the risk of provoking a severe asthma
attack, and few doctors use it unless there are compelling reasons to do so – such as demonstrating that someone’s asthma is due to an allergen encountered at work.
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.