Allergy testing

There are many substances we come into contact with that can trigger and cause allergies.  For example food, house dust mites, pollen, animals, moulds, drugs (eg antibiotics), lanolin, metals, solvents, household chemicals and many more.  If an allergic reaction is suspected, you will need to identify the exact allergen causing it.  To do so you should make an appointment to see your doctor, nurse or health visitor who will be able to advise you as to how to go about getting tested. 

Allergy testing can be done both privately and through the NHS and the type of test that is carried out will depend upon your symptoms or the condition of your skin.  The main tests that are available are:

• Skin prick test
• Blood test
• Patch test

Skin Prick Test

This is the most common allergy test and is generally the first test recommended when an allergy is suspected since it is cheap, safe and easy to perform with results being available quickly.  If it is performed correctly it has a high degree of accuracy and can be carried out safely on anyone from 4 months old.

The test is usually done on the forearm although in some cases it is carried out on the back.  The arm (or back) is coded with a marker pen for the allergens to be tested and then a drop of the allergen solution is placed by each code.  Up to 24 allergens can be tested at any one time.  The skin is then pricked through the drop using the tip of a lancet (small needle).

If there is a positive reaction to an allergen the skin around the needle prick becomes itchy with redness within a few minutes.  It will then develop a white swelling in the middle.  This is called a wheal and will reach its maximum size in about 15 to 20 minutes.  The reaction will fade within an hour.

Also included in skin testing is a negative and positive control. The negative control is a saline solution whilst the positive control contains histamine.  It is expected that there will be a reaction to the histamine however if there is a reaction to the saline then this will indicate that the skin is extremely sensitive and in these instances care needs to be taken.  In cases where there is a negative reaction to the histamine this could mean that medicines the sufferer is taking could block the response to the histamine and allergens.  In these incidences the test may need to be carried out again at a later stage after a period of not taking these medications.  However, if this is not the case, a negative skin test will generally mean that you are not sensitive to that allergen.

Blood tests

A blood test will measure the amount of Specific IgE circulating in the blood that the immune system has produced against a suspected allergen. A small sample of blood will be taken and then sent to a specialist laboratory for a RAST test.

Blood tests are generally used if:

• There is a risk of an anaphylactic reaction (in this instance skin prick testing is not suitable)
• There is extensive eczema which will make skin prick testing hard to do
• Antihistamine medication cannot be stopped due to the severity of the symptoms
• Unusual and rare allergens are suspected

The results generally take 7-14 days to come back and are graded from 0 (negative) to grade 6 (strong positive). The higher the grade, the more likely it is that there is an allergy to that allergen. Over 400 specific allergens can be tested for in blood tests.

Patch Test

This test is used in cases of contact dermatitis (eczema) where allergies are suspected. It can test for contact allergies to rubber, nickel, lanolin, hair dyes, cosmetics, perfume, preservatives and skin medications. 2-3 weeks before having a patch test you will need to stop using steroid creams and any eczema will need to be under control.

The allergens will be spread onto special aluminium discs and placed onto the skin (usually the back) and held in place by hypoallergenic tape. The patches will be coded, will remain on the skin for 48 hours and need to be kept dry. The patches will then be removed and the skin is examined with any redness or swellings being noted. The skin is then re-examined after a further 48 hours for any remaining local redness or swelling.

The interpretation of the results will be carried out by the hospital dermatology department.

When choosing how and where to be tested for allergies it is important to see a reputable and trained specialist. There are now many commercial allergy testing kits available including hair analysis tests, applied kinesiology and VEGA tests. Many are not recommended by doctors since there is little scientific evidence to support them.

Information written by the talkhealth medical panel

Last Reviewed: 7 February 2011
Next Review Date: 13 October 2012