Allergy Testing
Skin allergy testing is a method for medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.
The Process of Allergy Testing
During allergy testing a microscopic amount of an allergen is introduced to a patient's skin by various means:
- Prick test or scratch test: pricking the skin with a needle or pin containing a small amount of the allergen
- Patch test: applying a patch to the skin, where the patch contains the allergen
If as a result of the allergy testing an immuno-response is seen in the form of a rash, urticaria (hives), or (worse) anaphylaxis it can be concluded that the patient has a hypersensitivity (or allergy) to that allergen. Further allergy testing can be done to identify the particular allergen.
The "scratch test" as it's called, is still very commonly used as an allergy test. A similar test involving injecting the allergen is also used, but is not quite as common due to increased likelihood of infection and general ineffectiveness by comparison. There are other methods available to test for an allergy.
Skin end point titration
Skin end point titration (SET) uses an intradermal injection of allergens at increasing concentrations to measure allergic response. To prevent a severe allergic reaction, the test is started with a very dilute solution. After ten minutes, the injection site is measured to look for growth of wheal, a small swelling of the skin. Two millimetres of growth in 10 minutes is considered positive. If two millimetres of growth is noted, then a second injection at a higher concentration is given to confirm the response. The end point is the concentration of antigen that causes an increase in the size of the wheal followed by confirmatory whealing. If the wheal grows larger than 13mm, then no further injection is given since this is considered a major reaction.
Preparation for Allergy Testing
There are no major preparations required for allegy testing. At the first consult, the subject's medical history is obtained and physical examination is performed. All consumers should bring a list of their medications because some may interfere with the allergy testing. Other medications may increase the chance of a severe allergic reaction. Medications that commonly interfere with allergy testing include the following:
- Histamine antagonists like Allegra, Claritin, Benadryl, Zyrtec
- Antidepressants like Amitriptyline, Doxepin
- Antacid like Tagamet or Zantac
Consumers who undergo allergy testing should know that anaphylaxis can occur anytime. So if any of the following symptoms are experienced, a physician consultation is recommended immediately:
- Low grade Fever
- Lightheadedness or dizziness
- Wheezing or Shortness of breath
- Extensive skin rash
- Swelling of face, lips or mouth
- Difficulty swallowing or speaking
Contraindications in Allergy Testing
Even though allergy testing may sound like a benign procedure it does have some risks which include swollen red bumps (hives) may occur after the test. The hives usually disappear in a few hours after the test. In rare cases they can persist for a day or two. These hives may be itchy and are best treated by applying an over the counter hydrocortisone cream. In very rare cases one may develop a full blown allergic reaction. Physicians who perform allergy testing always have equipment and medications available in case an anaphylaxis reaction occurs. This is the main reason why consumers should not get allergy testing performed at corner stores or by people who have no medical training.
Allergy testing can be done on individual of all ages. However, there are times when an allergy test should not be done. Individuals who take medications for depression, gastric acidity or antihistamines should not undergo this test. In such cases, stopping the medications for allergy testing may not be worthwhile as one may develop symptoms from the untreated medical disorders. Individuals who have severe, generalised skin disease or an acute skin infection should not undergo skin testing. One needs uninvolved skin for allergy testing.
There are some individuals who are highly sensitive to even the smallest amount of allergen and in such scenarios, allergy testing is not recommended. Whenever the chances of an anaphylactic shock are high, the test is best avoided.
Besides skin tests, there are blood tests which measure a specific antibody in the blood. The IgE antibody plays a vital role in allergies but its levels in blood do not always correlate with the allergic reaction.
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