Allergy testing identifies specific triggers causing an immune system overreaction. The initial screening tool is typically the skin prick or scratch test, which checks for an immediate hypersensitivity reaction by exposing the skin to small amounts of allergens. A positive result involves a raised, red wheal forming within 15 to 20 minutes, indicating the presence of allergen-specific Immunoglobulin E (IgE) antibodies. However, a negative skin prick test does not eliminate the possibility of an allergy, especially if the patient’s clinical history suggests one. When this initial test is inconclusive, further investigations are necessary.
Immediate Follow-Up: Specific IgE Blood Testing
When a skin prick test is negative, the next step often involves checking for systemic evidence of an allergic response in the bloodstream. This measures the concentration of specific Immunoglobulin E (sIgE) antibodies, which the immune system produces in response to a particular allergen. These circulating antibodies are a direct sign of sensitization.
This laboratory analysis, known commercially as RAST or ImmunoCAP testing, exposes the patient’s serum to various allergen extracts. The test quantifies the sIgE antibodies bound to a specific allergen, providing a numerical value reported in kU/L (kilo-units per liter). This result correlates with the likelihood of a clinical allergy, helping to stratify risk without direct patient exposure.
Specific IgE blood testing is preferred when patients cannot stop taking antihistamine medications, which would suppress the skin reaction and cause a false negative. It is also the chosen method for individuals with extensive skin conditions, such as severe eczema, where the skin surface is too compromised for accurate prick testing.
Increasing Sensitivity: Intradermal Skin Testing
If both the initial scratch test and the sIgE blood test remain inconclusive, physicians may proceed to intradermal testing. This method uses a significantly increased concentration of allergen exposure compared to the prick test. It is frequently employed when investigating potential allergies to drugs, insect venoms, or when a high suspicion of allergy persists despite negative initial screening results.
The intradermal procedure involves carefully injecting a tiny volume (0.02 to 0.03 milliliters) of a highly diluted allergen solution directly into the dermal layer of the skin. This precise injection raises a small bump, or bleb, bypassing the outermost layer and allowing the allergen to interact directly with local immune cells. A reaction is judged by measuring the diameter of the resulting wheal and flare 15 to 20 minutes after the injection.
Because the intradermal method maximizes the chance of triggering a localized IgE-mediated response, it is highly sensitive. However, this process carries an elevated risk of inducing a systemic reaction in highly sensitized individuals. Therefore, intradermal testing must always be conducted under close medical observation, ensuring immediate access to emergency medications.
Confirming Diagnosis: The Oral and Drug Challenge
When screening tests are negative, conflicting, or do not align with the patient’s clinical history, the controlled challenge test is the definitive step. This procedure is considered the gold standard for allergy diagnosis because it directly tests the patient’s reaction to the suspected trigger. The goal is to definitively prove or disprove the clinical allergy by observing a reaction upon actual exposure.
In an Oral Food Challenge (OFC), the patient is given the suspected food allergen in small, gradually increasing doses over several hours. The food is often disguised or mixed with other ingredients as part of a single-blind protocol. Medical staff continuously monitor the patient for any signs of an allergic reaction, from mild itching to severe anaphylaxis.
Drug challenges follow a similar dose-escalation protocol, ensuring the body is exposed to a minimal dose first before proceeding to larger amounts. This slow, monitored exposure minimizes the risk of a sudden, severe reaction while allowing the immune system to display a response. If the patient tolerates the full therapeutic dose of the drug or the typical serving size of the food without symptoms, the allergy is considered disproved.
The most rigorous form is the double-blind, placebo-controlled food challenge (DBPCFC). In this test, neither the patient nor the staff knows if the dose contains the actual allergen or an inert placebo. This strict control eliminates psychological bias, providing the most accurate confirmation of an allergy. These challenges are always performed in a specialized clinic or hospital setting equipped to handle immediate anaphylaxis, including epinephrine and resuscitation equipment.