Patch testing is a method of determining whether a given substance is causing contact dermatitis, an allergic reaction affecting the skin. Patch tests can detect both immediate and delayed reactions; the latter can take several days to develop.
In a patch test, the doctor will apply the suspected allergen to a patch and then place it on the skin for 48 hours. During the test, the doctor may use 20 to 30 extracts of known allergens including preservatives, latex, medications, hair dyes, perfumes, resins and metals. The patient should avoid bathing and strenuous activities while wearing the patches.
At the end of the test, the patient returns to the doctor’s office to get the patches removed. The doctor will then examine the patch sites for signs of an allergic reaction.
How reliable is patch testing?
While generally reliable, patch testing is not foolproof. Sometimes there will be a false positive, which indicates an allergy when there is not one. The patient can also have a false negative in which the test fails to indicate an allergy that actually exists.
What causes a false negative result?
Some allergens can take longer than 48 hours to trigger a response. If the doctor is testing for any such allergens, he should have another test reading a day or two after the patches have been removed. Elderly patients are particularly prone to delayed reactions and are therefore likely to have false negative results.
Sometimes the false negative is due to a mistake made during the testing. The extract might be too weak, or the doctor might have used too little of it. Medications or therapies that suppress the immune system can make the patient’s skin unresponsive to the allergen. Sun exposure can also make the skin less responsive than normal.
What can cause a false positive result?
Some allergens can cause irritation in people who are not truly allergic to them. Formaldehyde and some metals fall into this category. The doctor may have also used an extract that was too strong. There can also be a spillover effect from a nearby positive result, especially a very strong one. Multiple positive results that occur more or less simultaneously can also cause false positives in nearby patch sites.
Patients who have inflamed or irritated skin while undergoing the patch test are likely to get false positive results.
What will happen if there is a false positive or false negative result?
In either case, the doctor will administer the patch test again to determine the veracity of the results. In the case of a false positive result, he will use extracts with lower concentrations of the allergen. He may also perform other, additional tests.