Skin prick tests


Skin allergy testing is a method for medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.

A microscopic amount of an allergen is introduced to a patient’s skin.

Techniques include: pricking the skin with a needle or pin containing a small amount of the allergen, scratching with a deep dermic scratch, intradermic test with tiny quantity of allergen is injected under the dermis wth a hypodermic syringe. a superficial skin scraping to remove the superficial layer of the epidermis, and patch test: applying a patch to the skin, where the patch contains the allergen.

If an immuno-response is seen such as a rash, urticaria, or anaphylaxis it can be concluded that the patient has an allergy to that allergen.

The skin scratch test is not very commonly used due to increased likelihood of infection.

The skin scrape test is painless, does not leave residual pigmentation and does not have a risk of infection.

Positive reactions reveal that the skin will become raised, red and appear itchy.

Larger wheals indicate more sensitivity to that particular allergen.

A negative test does not conclusively rule out the presence of an allergy, as the concentration of the allergen is inadequate.

A positive skin prick test is about 50% accurate, so a positive skin prick test alone is not diagnostic of food allergies.

This test is done in to identify allergies to pet dander, dust, pollen, foods or dust mites.

Intradermal injections are done by injecting a small amount of allergen beneath the skin surface to assess allergies to drugs like penicillin or bee venom.

To ensure that the skin is reacting appropriately, all skin allergy tests are also performed with proven allergens like histamine or glycerin.

The majority of people react to histamine and not to glycerin.

Patch test uses a large patch which has different allergens on it, and it is applied onto the skin, usually the back.

The allergens on the patch include: latex, medications, preservatives, hair dyes, fragrances, resins and metals.

The intradermal test, also called an intradermal skin end point titration (SET)

Also called an intradermal test, the skin end point titration (SET) uses intradermal injection of allergens at increasing concentrations to measure allergic response.

The intradermal test is initiated with very dilute solutions to prevent a serious allergic reaction.

Allergenic extracts, saline, and histamine  are placed  percutaneously and the resulting wheals are compared.

After 10 minutes following the initial injection, measurement of the swelling (wheal) at 2 mm indicates a positive reaction, and a second injection at. a higher concentration is given to confirm the allergic reaction.

The likelihood of a positive oral food challenge increases is serum specific IgE  levels or skin pricking wheal size increase.

The concentration of antigen that further increases the size of the wheal is the endpoint.

A wheal of 13 mm indicates a major allergic reaction.

Medications that interfere with skin testing include: antihistamines, antidepressants,

Histamine antagonists like Allegra, Claritin, Benadryl, Zyrtec Antidepressants like Amitriptyline, Doxepin Antacid like cimetidine or ranitidine

Patients who undergo skin testing may experience anaphylaxis at occur anytime.

Hives may occur after the test.

Skin testing can be done on individuals of all ages.

Patients on medications for depression, gastric acidity or antihistamines should not undergo this test, nor should individuals with severe generalized skin disease as the skin should be uninvolved with rash.

Whenever the chances of an anaphylactic shock are high as in highly allergic individuals , the test is best avoided.

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