Skin Prick test | Procedure of scratch test | Dermatology

Skin Prick test (scratch test)

What is a skin prick test?

The skin prick test is a diagnostic procedure performed to identify allergens (called allergens) that cause different types of allergic diseases. Also known as a scratch test, this test helps to check for allergic reactions to different types of allergens such as – fungi, mites, pollen, dust, epithelium, insects, and food.

Thus this test helps ensure that the substance that one touches, breathes, or eats causes symptoms of anxiety.

Is skin prick testing safe?

The skin prick test is very safe for most people, and it often does not need to be done in a hospital setting. Since such a small amount of allergen is used, the reaction is only very small. For most people, the worst thing to happen is a few itchy bumps for an hour or so. Some people will not be suitable for the skin prick test.

This includes people who have had severe allergic reactions in the past or people who have severe asthma. Skin prick tests are not usually done on pregnant women, people who take certain medications, or those who have very bad eczema.

Professionals who perform the skin prick test have emergency medicine and equipment standing ready for the very rare occasion when someone has a bad reaction. They can reverse the reaction if this happens.

Why the test is performed?

Allergy tests are done to find out which substances cause your allergy symptoms.

Your provider may order allergy skin tests if you have them:

  • Hay fever (allergic rhinitis) and asthma symptoms not well controlled with medication
  • Hives and angioedema
  • Food allergies
  • Skin rash (dermatitis), in which the skin becomes red, sore, or swollen after contact with the substance
  • Penicillin allergy
  • Venom allergy

Allergy to penicillin and related drugs is the only drug allergy that can be tested with skin tests. Skin tests for allergies from other medications can be dangerous.

The skin prick test can also be used to diagnose a food allergy. Intradermal tests are not used to test for a food allergy due to the high false-positive results and the risk of causing a severe allergic reaction.

How to prepare for the test?

Before any allergy test, the service provider will ask about:

  • Illnesses
  • Lifestyle
  • Where you live and work
  • Foods and eating habits

Sensitivity meds can change the aftereffects of skin tests. Your supplier will reveal to you which drugs to dodge and when to quit stepping through them before the examination.

Skin prick test procedure

Skin prick tests are usually done on the inner forearm. Any number of allergens can be tested, at least 3, 4, or up to about 25 allergens. Here is a brief overview of how the test is performed.

  • Clean the arm with soap and water or alcohol.
  • The forearm is coded with a skin marker pen that corresponds to the number of allergens tested. The distance between the marks should be at least 2 cm.
  • Next to each mark is placed a drop of allergy solutions.
  • A small prick is made through the drop into the skin using a sterile lancing needle. A new scalpel should be used for each allergen tested.
  • Excess allergen solution is wiped with a napkin.
  • Monitor skin reactions: If a reaction occurs, this should be done within 20-30 minutes.

In addition to the tested allergens, there should be a positive and negative control. The positive control, usually the histamine solution, should become itchy within a few minutes and then become red and swollen with “scars” in the center. The negative control, usually saline, should not show any response.

Results

Before you leave your doctor’s office, you’ll know the results of the skin prick or intradermal test. It may take several days or longer for the patch test to get results. A positive skin test means that you may be allergic to a specific substance. Larger shrivels usually indicate a greater degree of sensitivity. A negative skin test means that you may not be allergic to a specific allergen.

Keep in mind that skin tests aren’t always accurate. Sometimes it indicates an allergy when there isn’t a single allergy (false positive), or a skin test might not trigger a reaction when you’re exposed to something you’re allergic to (false negative). You may react differently to the same test done on different occasions. Or, you may react positively to a substance during the test but not interact with it in everyday life.

Your allergy treatment plan may include medications, immunotherapy, changes to your work environment or home, or dietary changes. Ask your doctor to explain anything you do not understand about your diagnosis or treatment. With test results that identify your allergens and a treatment plan to help you gain control, you’ll be able to reduce or eliminate allergy signs and symptoms.

Are there any risks involved?

Since a very small amount of the allergen is used, the reaction is also very small and clears up within a few hours.

Most of the time, the reaction can appear like a few itchy bumps.

  • Asthma patients tend to develop a severe allergic reaction or asthma attack within the next 24 hours. Therefore, they should always keep the physician informed of their condition and always carry inhalers and medications while this test is being performed.
  • Skin prick tests are not usually done on pregnant women, people who take certain medications, or those who have very bad eczema.
  • In rare cases, these allergy skin tests can lead to severe and immediate allergic reactions such as a whole-body allergic reaction (called anaphylaxis), which can be life-threatening.
  • This usually only occurs with an intradermal test/food allergy test. Therefore it is important to have skin tests done in a centre that has emergency equipment and appropriate medications and the health care provider is prepared to deal with all these emergencies.

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