From the Guidelines
Skin prick testing is generally the preferred method for diagnosing allergies due to its high sensitivity, immediate results, and lower cost, whereas RAST/specific IgE blood testing is better suited for patients with specific conditions that prevent skin testing, such as widespread skin disease or history of severe allergic reactions. When considering the diagnosis of allergies, it's essential to weigh the advantages and limitations of each testing method. Skin prick testing, also known as skin puncture test, involves placing small amounts of suspected allergens on the skin and pricking the surface to allow the allergen to enter, providing immediate results within 15-20 minutes 1. On the other hand, RAST/specific IgE blood testing measures the levels of IgE antibodies in the serum, which can be useful for patients who cannot undergo skin testing due to certain conditions, such as inability to stop antihistamines or very young age 1.
The sensitivity of specific IgE immunoassays, such as RAST, compared to skin prick/puncture tests has been reported to range from less than 50% to greater than 90%, with an average of around 70% to 75% for most studies 1. This variability highlights the importance of considering the clinical context and patient-specific factors when choosing a diagnostic test. Additionally, the presence of sIgE reflects allergic sensitization and not necessarily clinical allergy, emphasizing the need for careful interpretation of test results and consideration of clinical history 1.
In terms of specific guidelines, the NIAID-sponsored expert panel recommends performing an SPT to assist in the identification of foods that may be provoking IgE-mediated food-induced allergic reactions, but notes that the SPT alone cannot be considered diagnostic of food allergy 1. Similarly, the panel recommends sIgE tests for identifying foods that potentially provoke IgE-mediated food-induced allergic reactions, but alone these tests are not diagnostic of food allergy 1. Ultimately, the choice between skin prick testing and RAST/specific IgE blood testing should be based on individual patient needs and circumstances, with consideration of the potential benefits and limitations of each testing method.
From the Research
Comparison of RAST and Skin Prick Test
- The Skin Prick Test (SPT) is a commonly used method for diagnosing allergies, providing immediate results and being relatively inexpensive 2.
- RAST (RadioAllergoSorbent Test) is another diagnostic tool used to measure specific IgE antibodies in the blood, often used to confirm allergy diagnoses 3.
- A study comparing SPT, ImmunoCAP, and ISAC tests found that ImmunoCAP had a higher detection rate for nut allergy (71%) compared to SPT (56%) and ISAC (65%) 3.
- In the diagnosis of oral allergy syndrome, ISAC had a higher detection rate (88%) than ImmunoCAP (69%) and SPT (33%) 3.
Sensitivity and Specificity of Skin Prick Test
- A study evaluating the sensitivity and specificity of standardized allergen extracts in SPT found that the area under the receiver operating characteristic (ROC) curve was highly significant for all allergens and reference methods 4.
- The study found that specific IgE reference method resulted in the largest area under the curve (AUC) for all allergens, followed by previous SPT and case history 4.
- The sensitivity of SPT increased with increasing concentration, while specificity decreased 4.
Clinical Utility of Skin Prick Test
- SPT is considered the standard method and first-line approach for detecting allergic sensitization, with a positive predictive value ranging from 95-100% 2.
- However, SPT may not always coincide with clinical allergy, and results should be carefully interpreted in the context of clinical history 2.
- In cases where SPT results are negative, intradermal skin testing (IDST) may be useful in diagnosing respiratory allergies, particularly for house dust mite allergy 5.
Diagnostic Approach
- A combination of medical history, physical examination, and diagnostic tests such as SPT and serum food-specific IgE testing can provide a reliable diagnosis of food allergy 6.
- The physician-supervised oral food challenge remains the gold standard for food allergy diagnosis, but SPT and serum food-specific IgE testing can often provide a reliable diagnosis when used in conjunction with a careful medical history 6.