Allergy testing identifies specific triggers responsible for immediate hypersensitivity reactions, which are mediated by the antibody Immunoglobulin E (IgE). The Skin Prick Test (SPT) is generally employed as the initial screening method because it is fast, cost-effective, and highly sensitive for detecting IgE-mediated allergies. However, a negative SPT result does not definitively exclude an allergy, especially if the patient’s clinical history strongly suggests one. When the SPT is inconclusive or negative, more specialized testing methods are necessary to confirm or rule out the presence of allergy-causing antibodies.
Understanding the Skin Prick Test
The Skin Prick Test (SPT) introduces minute amounts of standardized allergen extracts into the uppermost layer of the skin. A small, sterile lancet gently punctures the skin surface through a drop of the extract, allowing it to interact with IgE antibodies bound to mast cells in the skin. If a person is sensitized to the allergen, the mast cells release chemical mediators like histamine, resulting in a localized, raised, itchy bump called a wheal, surrounded by a flare of redness, typically read after 15 to 20 minutes.
The SPT is prone to false-negative results under certain circumstances. A common cause of a false negative is the recent use of medications, such as antihistamines, which block the histamine response and suppress the skin reaction. Additionally, conditions like severe eczema or low skin reactivity in very young or elderly patients can interfere with the test’s ability to produce a visible reaction. In these cases, or when a high risk of systemic reaction makes skin testing unsafe, alternative diagnostic methods must be employed.
The Immediate Follow-up: Specific IgE Blood Testing
When the SPT is negative, the most common next step is to perform a specific IgE blood test. This method measures the actual quantity of allergen-specific IgE antibodies circulating in the bloodstream. A small blood sample is drawn and sent to a laboratory for analysis, where immunoassays determine the level of IgE antibodies corresponding to a suspected allergen.
The blood test offers several distinct advantages over skin testing. Since it is not performed on the skin, its results are not affected by medications like antihistamines, or by skin conditions such as widespread eczema. Specific IgE levels are typically quantified in kU/L, and a result higher than 0.35 kU/L generally suggests sensitization to that allergen. Higher IgE levels are associated with a greater likelihood of clinical reactivity. This test is also safer for individuals who may be at a higher risk of a severe allergic reaction from direct skin exposure.
Intradermal Testing and Its Role
If both the SPT and the specific IgE blood test are negative, but a strong clinical history of allergy persists, Intradermal Testing (IDT) may be used. The IDT is a variation of skin testing that is significantly more sensitive than the prick method. It involves injecting a very small amount of a highly diluted allergen extract just beneath the top layer of the skin, directly into the dermis.
Because the allergen is introduced deeper and in a more concentrated form than in the SPT, the IDT is capable of detecting a lower threshold of IgE sensitization. This increased sensitivity makes it particularly valuable for confirming suspected allergies to high-stakes allergens, such as penicillin or insect venoms. However, the IDT is not routinely used for common food or environmental allergies because its heightened sensitivity increases the risk of false-positive results and, in rare cases, a systemic allergic reaction.
Advanced Diagnostic Procedures
Beyond the initial skin and blood tests, other advanced procedures are reserved for complex or inconclusive cases. The most definitive method for confirming an allergy is the Challenge Test, which involves the controlled, supervised administration of the suspected food or drug allergen. This procedure is performed only in a medical setting with emergency equipment available, as it carries the risk of inducing a full allergic reaction.
For allergies that manifest as delayed skin reactions, Patch Testing is the appropriate diagnostic tool. This test is used exclusively for Type IV hypersensitivity, which is a cell-mediated response. Small quantities of suspected allergens, such as nickel, fragrances, or preservatives, are applied to patches placed on the skin, typically the back, and the site is evaluated after 48 to 96 hours for signs of allergic contact dermatitis.