Skin Prick Test - Indications, Procedure, and Result

Introduction

Skin prick test is a diagnostic method to identify the allergens responsible for causing allergic disease or symptoms. While patch testing is a valuable diagnostic tool for patients with allergies, contact dermatitis, skin prick testing is beneficial in detecting other types of allergies, such as those triggered by aeroallergens like hay fever, food, drugs, latex, and bee and wasp venom.

Performing a routine battery of allergen tests using skin prick testing is a fundamental and cost-effective approach to diagnosing allergic diseases. The procedure is straightforward, posings minimal risk, and is inexpensive.

What Are Allergens?

An allergen is a substance that triggers an allergic response in the body. In a skin prick test, the allergen is introduced beneath a layer of skin, prompting the immune system to react by producing antibodies to defend against what it perceives as a harmful substance.

When the allergen binds to a specific type of antibody, it triggers the release of chemicals, including histamine, that triggers the allergic reaction. During this process, several physiological changes occur, namely:

• Blood vessels dilate and become more permeable.

• Fluid leaks from blood vessels, causing redness and swelling.

• More mucus is produced, leading to congestion, a runny nose, and teary eyes.

• Nerve endings are stimulated, causing itching, a rash, or hives.

• Stomach acid production increases.

In severe cases, two additional reactions may occur:

• Blood pressure may drop due to dilated blood vessels.

• Airways may swell, and bronchial tubes constrict, leading to difficulty breathing.

What Are the Indications to Perform This Test?

Skin prick testing is a standard diagnostic tool for atopy, a condition characterized by an exaggerated immune response to environmental triggers and influenced by genetics. This condition typically manifests as one or more typical conditions, such as:

1. Asthma.

2. Eczema.

3. Hay fever (rhinitis).

Skin prick testing results can guide treatment options for asthma and hay fever patients, including desensitization to specific allergens, removal of pets or carpets, and avoiding certain foods. Skin prick testing is also helpful for patients with bee, wasp stings, and acute urticaria, especially if immunotherapy is considered. However, skin prick testing is generally not beneficial in investigating conditions such as:

1. Atopic dermatitis.

2. Chronic urticaria.

3. Angioedema.

4. Food intolerances.

5. Dermatitis.

6. Herpetiformis or nonspecific rashes.

How to Prepare for the Test?

Before undergoing an allergy test, it is essential to provide the allergist with comprehensive information about the allergy, such as when and where they occur and how the body reacts.

It is critical to refrain from taking antihistamines before the test, and the patient should inform the allergist of any antihistamines they regularly take. Depending on the medication's half-life, the patient may need to discontinue it for over a week. This also includes cold or allergy medications containing antihistamines combined with other substances.

Several other medications may also impact the skin prick test's accuracy, so it is vital to discuss this with the allergist and potentially avoid taking them before the test. On the day of the test, avoid applying lotion or perfume to the skin area where the test will be conducted.

Although a person may test positive for an allergen, they may not experience any allergy symptoms. False positives and negatives are also possible, and a false negative result could be hazardous because it may not detect the substance the person is allergic to, leaving him unaware of what to avoid. Nonetheless, it is still advisable to get tested because identifying the substances that trigger the allergies enables the patient to work with the doctor to develop a treatment plan to alleviate the symptoms.

What Is the Procedure for Performing the Test?

1. To conduct the skin prick test, the nurse will first sanitize the area of the skin to be tested with alcohol.

2. Then, they will mark the skin with a series of indicators to keep track of the different allergens and the skin's responses to them.

3. Next, a small amount of each allergen will be placed on the skin, and the nurse will use a light prick to let the allergen seep into the skin. Although this procedure is not typically painful, some people may experience mild irritation.

4. After administering the allergens, the patient must wait around 15 to 20 minutes for any reactions.

5. If allergic to a substance, they will likely develop a red, itchy bump on the spot where the allergen was placed, which will look like a mosquito bite surrounded by a red ring.

6. The nurse will evaluate and measure the reactions, and the bumps from the skin reaction will usually disappear within a few hours.

Skin prick testing is a safe procedure for individuals of all ages, including infants over six months old. Although rare, skin prick testing may trigger severe allergic reactions, particularly in people with a history of severe reactions or food allergies. The doctor will be prepared to identify and treat these reactions.

What Does Result Mean?

• Normal Result: If a skin prick test yields a negative result, as there were no skin reactions or responses to the allergen, indicating that the person is most likely not allergic to the substance. However, in rare cases, a person may still be allergic to the substance despite a negative allergy test result.

• Abnormal Result: If the test registers a positive result, as the patient developed a reaction to the substance, showing up as a red, raised area called a weal. Typically, a positive impact indicates that the symptoms are caused by exposure to that substance, with a stronger response implying a greater sensitivity to it. However, it is possible to test positive for a substance and not experience any issues with it in everyday life.

Although skin tests are usually accurate, a hefty dose of an allergen can cause a positive reaction, even in people who are not allergic. Based on the symptoms and test results, the healthcare provider will suggest lifestyle changes to avoid substances triggering the symptoms.

Are There Any Complications Associated With Tests?

To ensure accuracy in skin prick testing, placing the tests at a minimum distance of 2 cm is crucial to prevent allergen solutions from spreading between test sites. False-positive and false-negative results can occur due to various factors.

False-positive results can be triggered by a positive reaction from one test site affecting the neighboring site or an irritant reaction. On the other hand, false-negative results may occur due to the blocking effect of histamine by medications such as antihistamines. In addition, decreased skin reactivity in infants and elderly patients and diluted allergen extract (especially with foods) can also contribute to a false-negative result.

Conclusion

Skin prick testing is a safe, inexpensive, and effective diagnostic tool to identify allergens that cause allergic diseases. It is beneficial for detecting allergies triggered by aeroallergens like hay fever, food, drugs, latex, and bee and wasp venom. Skin prick testing is a standard diagnostic tool for atopy, a condition characterized by an exaggerated immune response to environmental triggers and influenced by genetics. The safe test can be performed on individuals of all ages, including infants over six months old.