The commonest type of skin test, most often used to identify the causes of inhaled allergy and immediate food allergy, is the skin prick test.  Certain kinds of allergic reaction; for example, allergy to certain drugs, and allergy to moulds, yeasts and fungi; may only show up using an intradermal test, an alternative to the skin prick test that is less commonly used in Europe than in the USA. A different type of test called a patch test is used when contact allergy is suspected, and in certain cases of drug allergy.

Skin prick testing

Click here   to see a video of a skin prick test being performed, 

This test involves placing separate drops of solutions of allergen extracts onto the skin of your forearm or back and then using a very fine lancet to prick through the drop into the skin. A positive test results in a small raised wheal with a red surrounding flare.  Results are known within fifteen minutes and the test is painless.  It may be necessary for you to stop certain medications such as anti-histamine medications before a skin prick test but you should only do this under the supervision of a doctor.  Oral or inhaled steroids do not interfere with skin prick tests.  

Performing an allergy skin prick test

A small drop of a solution containing a dilution of the substance under test is placed on the forearm, and a lancet is used gently to prick the skin through the solution.  If the patient is allergic to the allergen being tested, the skin on the arm will swell with fluid (the paler area), and get itchy and red in the surrounding area.  In the picture below, the reference tests are on the left; histamine (positive control) and diluent (negative control).  This method of testing is excellent for certain conditions and certain allergens, although it is less exact for others.  Careful comparison between the results of skin tests and the results of challenge tests, either by inhalation (bronchial challenge) or by ingestion (food challenge) have shown that the skin test may in some cases over-diagnose (false positive) or in some cases under-diagnose (false negative).  For this reason it is important that diagnosis based on the skin prick test should consider the result alongside the patients own experience, the clinical history.

It is also important to understand that food intolerance (delayed or 'hidden' food allergy) does not show up using this method, and will usually only show up by the use of an elimination diet.  Such 'false negative' results to skin prick tests probably occur because the reaction to the food being tested is either not an allergy at all (for example a toxic effect or an enzyme deficiency as in lactose intolerance) or it is an allergy, but one that works in a way that, as yet, we do not fully understand.

Skin Prick Testing for Drug Allergy
Currently, only limited tests are available to help in the diagnosis of drug allergy.  Allergy skin testing is available for penicillin and insulin. Skin testing is sometimes done for other medication, but is is often of limited accuracy.

Intradermal skin testing

This is more sensitive than the skin prick test in certain situations.  0.05 — 0.1ml of the substance is injected into the very outermost layer of the skin of the arm.  The result can be read at 20-30 minutes and in some cases at 24 hours and at 48 hours.

A positive reaction at 30 min is suggestive of immediate (type I) allergy and positive reaction at 48 hours suggests delayed allergy (type IV).

This test is done much less frequently than the skin prick test (although it tend to be more popular in the USA), because of a concern that it may give false positive results.  Also the risk of general anaphylaxis, although still very low, is a little higher than in prick testing because of the larger amount of allergen injected.  However, once better validated, it may be a way to diagnose certain kinds of allergic reaction; for example, allergy to certain drugs, and allergy to moulds, yeasts and fungi.

Patch (contact) testing

Patch testing can be used to assist in the diagnosis of contact eczema (contact dermatitis) and may also be useful to assist in the diagnosis of certain types of drug allergy.  In a recent development, patch testing has also been used to try to assist in the diagnosis of delayed food allergy; a condition where the standard skin prick test may give a falsely negative reading.  However, this form of testing remains to be fully evaluated and is only a research technique at present.

Contact eczema (contact dermatitis) is caused by an allergic reaction to something that touches the skin.  A very wide range of substances is now known to be capable of inducing such reactions.  A list of common contact allergens is given below.  From the nature of these it can be seen that particular occupations pre-dispose the workers to contact sensitivity.  

The diagnosis of contact dermatitis (for example, a skin allergy to a cosmetic, a metal such as nickel, or a chemical in rubber) requires a 'patch test'.  This is because the usual allergy antibody (IgE) is not involved, and a skin prick test or allergy blood test would not pick it up.  The reaction is thought to be related to a response of the white blood cell rather than an antibody response, and the reaction therefore take a day or two to develop.  Several substances to which the patient may turn out to be sensitive are placed into tiny aluminium chambers mixed into a gel and these are then attached to the skin for 48 to 72 hours to determine whether an area of itchy rash develops, the sign of contact allergy.  Patch tests responses need to be interpreted with care to distinguish which show an allergic and which show an irritant reaction.

To find out the cause of contact eczema in the individual patient can require close and careful detective work.  First the doctor needs to take a very careful history including information about the patient's work and hobbies.  A range of standard contact 'patch' tests is then done supplemented, if necessary, by additional tests with items to which the patient becomes exposed on a regular basis.  For example, a hairdresser with contact eczema would have patch tests to the various hair treatment chemicals used in her job.  The list below includes some of the most common skin sensitisers and shows where they are found.

Common contact allergens

Skin testing for drug allergy

When patients suffer from unwanted effects of drugs, they tend to call the reaction a drug allergy.  However, in less than 10% of cases is a true (immune-mediated) drug allergy the cause. The other cases are called adverse drug reaction. 

When the circumstances suggest true drug allergy, skin and other allergy tests are only useful in a minority of patients. For the majority of cases, if the diagnosis needs to be confirmed, it has to be done by oral challenge in a hospital setting.

The skin prick test, the intradermal test and the patch test can all be helpful in certain cases of suspected drug allergy.