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Allergy patch testing is used to screen substances to determine the cause of an allergic skin reaction. Patches with different suspected irritants (allergens) are applied to a person’s back and left in place for 48 hours. The skin is then examined for signs of a hypersensitive reaction.
Patch testing detects allergic contact dermatitis, a type IV hypersensitivity reaction. It cannot identify the cause of urticaria (hives) or food allergy. Prick testing is another method used to test for a different type of allergy (type I) associated with hay fever, asthma and contact urticaria. It involves pricking the skin with a needle in order to test the blood and can be read after just 20 minutes.
The types of allergens tested include hair dye, preservatives, cosmetics and medications. A standard set of allergens (such as those described in the European Standard Series of allergens) are applied to almost every patient as these are the most common causes of dermatitis (in 85% of cases). A variety of other compounds are also applied, and these are tailored to the individual, who may bring their own materials from home or work. The optimum concentration for each substance has been predetermined, in order to induce an allergic reaction in susceptible skin without irritating skin that is not allergic.
Patch testing can also be helpful in determining the cause and pathophysiological mechanisms of a cutaneous adverse drug reaction (CADR). The clinical relevance depends on the drug involved and type of CADR investigated, ideally: generalized eczema, systemic contact dermatitis, maculopapular rash, acute generalized exanthematous pustulosis and fixed drug eruption. Approximately 32-50% of drug patch tests are positive in patients who have developed a CADR.
The general format for a patch test is as follows;
Interpretation of skin reactions requires a good deal of experience and training. It is important to distinguish between an irritation and an allergic reaction. The former occurs after the patch is removed but will reduce over the following day, whereas the latter develops over several days and is more prominent on day five rather than immediately following patch removal. The types of reactions observed may include:
Patch tests may not always be conclusive. False negatives (no reaction despite an underlying allergy) and false positives (indicating an allergy when there isn’t one) do occur. Patients can react differently to substances during a test than in everyday exposure, or to the same test performed at different times.
A positive reaction causes an area of itchy, active dermatitis at the site of patch application. This is usually treated with topical steroid. Other potential reactions include:
Medications can also interfere with the results of a patch test. Different drugs can either suppressor increase the risk of developing allergic reactions, which reduces the accuracy of the test. These include: antihistamines, tricyclic antidepressants, heartburn medications and the asthma medication omalizumab. Patients may need to stop taking medications for up to 10 days before a patch test, depending on how quickly they are cleared from the system.
By understanding the cause of their allergic dermatitis, patients can proceed with reducing symptoms or eliminating potential triggers of their symptoms. Treatment management plans may consist of medications, immunotherapy, dietary changes and/or alterations to their work or home environment.