Allergy skin tests are a quick, effective, cost-effective, and safe way to identify potential allergies in individuals.

Introduction

The skin is often considered a reliable indicator of allergic reactions. Allergy tests involving the skin, such as the patch test or the skin prick test, can reveal sensitivities to various allergens. 

Allergic reactions on the skin might indicate that other parts of the body, such as the eyes, nose, or lungs, could also be sensitive to the same allergens. Understanding skin reactions is crucial for doctors to determine an appropriate treatment plan. 

By identifying the allergens causing the reactions in an individual, healthcare professionals can develop strategies to manage and alleviate the allergy symptoms. For this, they might recommend either avoiding substances suspected to cause allergy or suggest immunotherapy if necessary. Larn more about these tests at Fortis Healthcare.

Histamine and antibody

The visible reaction on the skin, called a wheal and flare response, occurs because of the release of histamine and other substances. The response is visually clear and can be measured (or quantified). 

The purpose of an allergy test is often to measure the levels of the IgE antibody, which plays a key role in allergic reactions. 

Two common methods to measure IgE levels include direct quantification in the lab and skin prick testing (SPT). A skin prick test gives accurate results in 15–20 minutes. Its quick turnaround time helps healthcare professionals make timely interventions and ensure effective patient care. 

What is skin allergy?

Skin allergy refers to a reaction to certain chemicals. Over 3,000 chemicals are known skin allergy-causing agents. About 50 of these chemicals account for 80% of clinical cases. The process of skin allergy development involves an initial sensitization phase where allergens interact with immune cells, activating memory cells specific to the allergen. The reaction typically manifests 12–72 hours after exposure to the allergen. 

Commonly affected areas include the hands, face, arms, and legs. The reactions can be triggered by cosmetics used for makeup, metals from jewelry, and rubber products such as gloves or watch straps.

Once the allergen is identified, individuals are usually advised to avoid exposure to it. Alternatively, the doctors may prescribe corticosteroids that help manage allergic symptoms even after allergen exposure.

How is a skin allergy tested?

To do a skin allergy test, extracts of several potential allergens are applied to the skin. The procedure is carefully executed through the following steps:

1. Preparation:
   • The skin is cleaned with a solution containing 70% alcohol.
2. Marking allergen application locations:
   • Allergen application locations are identified and marked on the forearm or back, ensuring proper spacing to prevent false positive reactions.
3. Allergen application:
   • Each allergen is pricked into the skin in a strict concentration of either 1–10 or 1–20 g/L. A metal lancet is dipped in a drop of an allergen extract and pressed against the skin for a few seconds.
   • This process is repeated for each allergen, ensuring that each applied drop contains only one allergen.
4. Timing and reading:
   • A timer is set for 15–20 minutes.
   • The test sites, including a control site (no allergen), are then assessed for any visible reactions.
5. Positive result indication:
   • Allergy is confirmed by a positive result, indicated by the appearance of a skin bump or wheal of at least 3 mm in diameter. 

Interpreting the results

To interpret the results, the size of the bump for each allergen is measured. The result is positive – that is, allergy is confirmed -- if the wheal has a diameter of 3 mm or more. This criterion, particularly when exceeding a specific value (wheal area: 17 mm²), provides a more accurate estimate compared to other measurements. 

Benefits of skin allergy test

1. Minimally invasive and cost-effective:
   • Skin prick test is a minimally invasive, cost-effective procedure that gives immediate results. It has excellent reproducibility with very few false-negative results. 
2. Versatility and applicability:
   • The skin prick test is used for diagnosing allergic conditions, such as rhinoconjunctivitis, asthma, urticaria, anaphylaxis, atopic eczema, and suspected reaction to certain food and drugs.
   • It can be used in epidemiologic studies to determine sensitization rates and regional differences.
3. Immediate results and safety:
   • Results are available within 15–20 minutes, enabling quick decision-making.
   • This is a safe procedure with little to no complication risk.

A few considerations before the test

Individuals with asthma should be well-controlled before the test. For those with a history of severe allergic reactions, diluted solutions are used. 

Individuals with conditions such as extensive eczema or urticaria or those taking medications are not suitable for SPT. For such persons, the in vitro measurement of specific IgE antibodies is considered an important complementary tool.

What are the risks?

While skin allergy tests are generally safe, there have been rare instances of severe allergic reactions. In some cases, individuals might experience serious systemic reactions that affect the entire body, potentially leading to breathing difficulties. 

Healthcare professionals performing the test should have emergency-related equipment, including epinephrine, readily available during testing.

Certain allergens, such as food allergens and latex, carry a higher risk of causing systemic reactions than others. Such reactions typically occur within 30 minutes of exposure, underscoring the importance of patient alertness during this timeframe and promptly reporting any adverse effects.

Allergens such as peanuts, tree nuts, and shellfish might pose a high risk. If a patient suspects severe allergies to specific substances, additional precautions and specialized testing methods may be necessary.

Individuals taking beta blockers or ACE inhibitors may be at a heightened risk due to low responsiveness to epinephrine. They should inform healthcare professionals about their medication history.

Skin prick test is relatively contraindicated during pregnancy due to the possibility of inducing systemic allergic reactions that could affect uterine contractions. Pregnant women should discuss their problems thoroughly with their healthcare providers to understand any potential risks.

SPT may also not suit patients with severe eczema or those taking medications like antihistamines, antidepressants, or calcineurin inhibitors. They may be advised to opt for alternative testing methods.

Chronic illnesses, such as renal failure or cancer, can affect skin test reactivity, and exposure to UV-B radiation might affect test results. 

Patients should also be alert and check the stability and expiration date of allergen extracts used in SPT. This emphasizes the need for proper storage conditions to ensure accurate results.

The inclusion of positive and negative controls during testing is essential for accurate results. Patients can inquire about these controls.

Specific conditions, such as oral allergy syndrome induced by raw foods, involve pricking the skin with the allergen. In such cases, patients should be made aware of the specific procedures adopted for their conditions.

When are intradermal skin allergy tests done?

Intradermal tests are typically done if the results of the skin prick test are negative and there is a strong suspicion of allergy based on clinical history.

These tests are employed when there is a need for increased sensitivity in detecting allergic reactions, particularly in situations where skin prick tests might yield false negative results or when heightened sensitivity is required for accurate diagnosis.

These tests are used when there is a risk of serious systemic allergic reactions, such as anaphylaxis, Hymenoptera venom sensitivity, or sensitization to medications, or in cases where an allergen is considered historically relevant.