Skin allergy test

Skin allergy testing or skin prick test (SPT) is a method for medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.

This article is about the method for medical diagnosis of allergies. For other uses, see Scratch test (disambiguation).
Skin allergy test
SynonymsSkin prick test
Purposediagnosis of skin allergies

Methods

A microscopic amount of an allergen is introduced to a patient's skin by various means:[1]

A person receiving a skin allergy test
  • Skin prick test: pricking the skin with a needle or pin containing a small amount of the allergen.[2]
  • Skin scratch test: a deep dermic scratch is performed with help of the blunt bottom of a lancet.[3]
  • Intradermic test: a tiny quantity of allergen is injected under the dermis with a hypodermic syringe.
  • Skin scrape Test: a superficial scrape is performed with help of the bovel of a needle to remove the superficial layer of the epidermis.[4]
  • Patch test: applying a patch to the skin, where the patch contains the allergen

If 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 testing can be done to identify the particular allergen.

The "skin scratch test" as it is called, is not very commonly used due to increased likelihood of infection. On the other hand, the "skin scrape test" is painless, does not leave residual pigmentation and does not have a risk of infection, since it is limited to the superficial layer of the skin.

Some allergies are identified in a few minutes but others may take several days. In all cases where the test is positive, the skin will become raised, red and appear itchy. The results are recorded - larger wheals indicating that the subject is more sensitive to that particular allergen. A negative test does not conclusively rule out an allergy; occasionally, the concentration needs to be adjusted, or the body fails to elicit a response.

Immediate reactions tests
Skin testing on arm

In the prick, scratch and scrape tests, a few drops of the purified allergen are gently pricked on to the skin surface, usually the forearm. This test is usually done in order to identify allergies to pet dander, dust, pollen, foods or dust mites. Intradermal injections are done by injecting a small amount of allergen just beneath the skin surface. The test is done to assess allergies to drugs like penicillin[5] or bee venom.

Skin testing on back

To ensure that the skin is reacting in the way it is supposed to, all skin allergy tests are also performed with proven allergens like histamine, and non-allergens like glycerin. The majority of people do react to histamine and do not react to glycerin. If the skin does not react appropriately to these allergens then it most likely will not react to the other allergens. These results are interpreted as falsely negative.[6]

Delayed reactions tests
See also: Patch test
Patch test

The patch test simply uses a large patch which has different allergens on it. The patch is applied onto the skin, usually on the back. The allergens on the patch include latex, medications, preservatives, hair dyes, fragrances, resins and various metals.

Skin end point titration

Also called an intradermal test, this skin end point titration (SET) uses intradermal injection of allergens at increasing concentrations to measure allergic response.[7] To prevent a severe allergic reaction, the test is started with a very dilute solution. After 10 minutes, the injection site is measured to look for growth of wheal, a small swelling of the skin. Two millimeters of growth in 10 minutes is considered positive. If 2 mm 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 13 mm, then no further injection are given since this is considered a major reaction.

Preparation

There are no major preparations required for skin 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 testing. Other medications may increase the chance of a severe allergic reaction. Medications that commonly interfere with skin testing include the following:

Consumers who undergo skin 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

Even though skin testing may seem to be a benign procedure, it does have some risks, including swollen red bumps (hives) which 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.[8] In very rare cases one may develop a full blown allergic reaction. Physicians who perform skin test always have equipment and medications available in case an anaphylaxis reaction occurs. This is the main reason why consumers should not get skin testing performed at corner stores or by people who have no medical training.

Antihistamines, which are commonly used to treat allergy symptoms, interfere with skin tests, as they can prevent the skin from reacting to the allergens being tested. People who take an antihistamine need either to choose a different form of allergy test or to stop taking the antihistimine temporarily before the test. The period of time needed can range from a day or two to 10 days or longer, depending on the specific medication. Some medications not primarily used as antihistamines, including tricyclic antidepressants, phenothiazine-based antipsychotics, and several kinds of medications used for gastrointestinal disorders, can similarly interfere with skin tests.[9]

People who have severe, generalized skin disease or an acute skin infection should not undergo skin testing, as one needs uninvolved skin for testing. Also, skin testing should be avoided for people at a heightened risk of anaphylactic shock, including people who are known to be highly sensitive to even the smallest amount of allergen.[10]

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.[11]

There are many alternative health care practitioners who perform a variety of provocation neutralization tests, but the vast majority of these tests have no validity and have never been proven to work scientifically.

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See also

References
  1. Bernstein IL, Li JT, Bernstein DI, Hamilton R, Spector SL, Tan R, et al. Allergy diagnostic testing: an updated practice parameter. Ann Allergy Asthma Immunol 2008; 100:S1-148.
  2. Basomba A, Sastre A, Pelaez A, Romar A, Campos A, Garcia-Villalmanzo A. Standardization of the prick test. A comparative study of three methods. Allergy 1985; 40:395-9.
  3. Indrajana T, Spieksma FT, Voorhorst R. Comparative study of the intracutaneous, scratch and prick tests in allergy. Ann Allergy 1971; 29:639-50.
  4. Olivier CE, Argentão DGP, Santos RAPG, Silva MD, Lima RPS, Zollner RL. Skin scrape test: an inexpensive and painless skin test for recognition of immediate hypersensitivity in children and adults. The Open Allergy Journal 2013; 6:9-17. Link Archived August 3, 2013, at the Wayback Machine
  5. Marwood, Joseph; Aguirrebarrena, Gonzalo; Kerr, Stephen; Welch, Susan A; Rimmer, Janet (2017-10-01). "De-labelling self-reported penicillin allergy within the emergency department through the use of skin tests and oral drug provocation testing". Emergency Medicine Australasia. 29 (5): 509–515. doi:10.1111/1742-6723.12774. ISSN 1742-6723. PMID 28378949. S2CID 1937840.
  6. American Academy of Allergy Asthma & Immunology: What is Allergy Testing?, Retrieved on 2010-01-20. |archive-url=https://web.archive.org/web/20120120175201/https://www.aaaai.org/conditions-and-treatments/library/at-a-glance/allergy-testing.aspx |archive-date=20 January 2012
  7. Skin Test End-Point Titration at the US National Library of Medicine Medical Subject Headings (MeSH)
  8. Skin Testing and Allergy Injection Treatment for Allergies and Asthma - The University of Arizona Health Sciences Center, Retrieved on 2010-01-20.
  9. Allergy Testing - August 15,2002 - American Family Physician, Retrieved on 2010-01-20.
  10. Skin test for Allergy, Retrieved on 2010-01-20.
  11. Skin Testing Basic Information, Retrieved on 2010-01-20. Archived January 14, 2010, at the Wayback Machine
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