Allergy testing – what do skin tests mean?
Skin testing and blood tests for IgE (RAST tests) are the only proven tests for allergy.  We use both skin testing (skin prick testing) for allergy and specific IgE blood tests.

Skin tests are a safe and easy way to investigate allergy.  Skin testing should be interpreted by a doctor who has experience with allergy and allergy testing, in combination with a careful history of symptoms from the patient. 

Are skin tests and blood tests for allergy accurate?
Skin tests can prove a diagnosis of allergy, but not always. Skin tests for some allergies are more accurate than others.  Furthermore, the size of the skin test reaction also increases the chance that the test is a true positive result for allergy.

The most important thing however, is whether the skin test result matches the symptoms of allergy from the patient.  If it does, then the diagnosis may be clear.  If it does not, then the skin tests can be a "false positive".

A positive skin test result in a patient with no symptoms to suggest allergy may in fact be a false positive.  Alternatively, if a patient has a very clear history to suggest allergy to a particular substance and the testing is negative, it may be a false negative test. 

As such, the interpretation of skin prick tests (and IgE blood or RAST allergy tests), can only be done in combination with a history of a patient's symptoms to see if these are consistent with allergy as well.  This is one of the most important aspects of allergy testing and allergy diagnosis.

Oral food challenge
For food allergy testing or medication allergy testing a "challenge test", also known as an oral food challenge, can be considered.  This should only be considered by an experienced Immunologist/Allergist.  The advantage of an oral food challenge is it can definitively disprove allergy.  For patients this can be very helpful, to free up this worry from their life. 

An understanding of skin test and allergy blood tests, as well as a good clinical knowledge of probabilities and good communication and discussion with a patient about their preferences, helps guide any decision whether to progress to an oral challenge. 

Which allergy test is best, skin test, blood test, or food challenge?
This actually depends on the clinical situation and the allergy in question.  Sometimes a blood test is preferable, sometimes a skin test gives more information, sometimes both are necessary, and in some clinical situations actually neither should be done.  Any broad statements about one type of testing being better than another will be incorrect over simplifications.  Food challenge does give the definitive answer, however should only be done if clinical history and skin tests/blood tests suggest it is safe.

Did you know?

• Hair testing has no scientific basis and no evidence to support it.  Hair testing is not an accepted part of medical practice and is not supported by major international allergy organisations. 
• This is also true for sending IgG blood tests to foreign countries. Use of alternative sham allergy testing like Hair tests and IgG testing is unfortunately common in New Zealand, spreading false information which actually can be very harmful. Please see this link to a commerce commission ruling regarding these false allergy practices.
• Also, large screens of numerous allergy tests should not be ordered either because they greatly increase the chances of false positive test results.

Harms of unproven allergy tests
Unproven allergy tests can be very harmful.  Below is a list of some potential harms of hair testing and IgG testing.

• Missed non-allergic diagnosis.  For example, if symptoms were due to another disease, not allergy, and the disease was serious (for example cancer). If missed this could be very harmful.  Giving a false allergy diagnosis might stop a patient pursuing investigation for the real cause.
• Allergic reaction.  Hair testing for allergy and IgG blood tests for allergy have no reliable evidence to support their use.  A risk is these tests stating a patient is not allergic to a substance when in fact they are.  In the case of serious allergic reactions, such as urticaria, anaphylaxis or angioedema, this could be very dangerous or even fatal.  This is why accepted testing methods which have extensive evidence to support their efficacy should be used i.e. skin prick tests, specific IgE or RAST blood tests, followed by oral challenge if considered safe and appropriate by an experienced Allergist / Immunologist.
• Harms of unnecessary avoidance diets.  For people with food allergies it can be very stressful avoiding foods.  Did you know that rates of depression in carers for children with allergy are high?  Also, patients can become significantly malnourished due to food avoidance.  Food anxiety and food aversion can develop.  For young children the stigma of not being able to join in at birthday party meals and being an "allergy kid" can be very significant.  For these reasons unnecessary avoidance diets are very harmful.  Causing these problems unnecessarily with unproven and unscientific allergy tests is unfortunately commonplace in Auckland.
• Placebo then nocebo.  Many people embark on an unnecessary avoidance diet and feel better.  This is due to a placebo effect.  Unfortunately, this tends to wear off by a few months, leaving a patient feeling confused.  The avoidance diet seemed to help, but now symptoms have returned.  It is natural to think "It must be something else I need to avoid as well".  As a result, a whole cycle of ongoing food avoidance continues, yet symptoms intermittently continue as well.  This is the "Nocebo" effect (effectively like a placebo effect but actually negative and harmful).  This is another risk of "sham" unproven allergy tests such as hair testing or IgG testing.

Dr Andrew Baker Immunologist Allergy Specialist Auckland  03/08/13.