From the Guidelines
Food allergy blood panels, such as RadioAllergoSorbent Test (RAST) or ImmunoCAP, have significant limitations and should not be relied upon as the sole diagnostic tool for food allergies, as they can only identify potential allergens and not definitively diagnose a true food allergy. These tests measure IgE antibodies to specific foods, but a positive test result indicates sensitization to a food, not necessarily a clinical allergy, as noted in the guidelines for the diagnosis and management of food allergy in the United States 1. The presence of sIgE reflects allergic sensitization and not necessarily clinical allergy, and several studies comparing the quantity of sIgE to oral food challenges have reported that the greater the levels of sIgE, the higher the probability that ingestion of the food will lead to an allergic reaction, but the predictive values varied from one study to another 1.
Some key limitations of food allergy blood panels include:
- False positives are common, particularly in people with other allergic conditions like eczema or asthma, potentially leading to unnecessary food restrictions
- False negatives can occur, missing genuine allergies
- Blood tests cannot predict the severity of allergic reactions or detect non-IgE mediated food allergies such as those causing gastrointestinal symptoms
- The correlation between antibody levels and clinical symptoms is often poor
For these reasons, blood test results should always be interpreted by an allergist in conjunction with the patient's clinical history, and oral food challenges remain the gold standard for definitively diagnosing food allergies, as noted in the work group report on oral food challenge testing 1. Patients should never make significant dietary changes based solely on blood test results without professional guidance. Additionally, the results from different laboratories or different assay systems may not be comparable, and the predictive values associated with clinical evidence of allergy for one test method cannot be applied to other test methods 1.
From the Research
Limitations of Food Allergy Blood Panels
The limitations of a food allergy blood panel, also known as a RadioAllergoSorbent Test (RAST) or ImmunoCAP, for diagnosing food allergies include:
- High false positive rates, leading to unnecessary dietary restrictions and increased healthcare costs 2
- Lack of validation as screening tests, making them unreliable for diagnosing food allergies 2
- Difficulty in interpreting results, particularly in the absence of a clear clinical history of an allergic reaction 3
- Limited ability to diagnose food allergies in the absence of IgE sensitization, which can occur in rare cases 3
- Need for additional testing, such as oral food challenges, to confirm a diagnosis of food allergy 4, 3, 5, 6
Diagnostic Challenges
The diagnosis of food allergy can be challenging, and blood panels are not sufficient on their own to make a diagnosis. A comprehensive diagnostic approach is necessary, including:
- Clinical history and physical examination 5
- Food allergy screening tests, such as skin tests and serologic tests 5
- Diagnostic elimination diets and oral food challenges, which are considered the gold standard for diagnosing food allergies 5, 6
- Novel diagnostic methods, such as component testing and identification of optimal cutoffs, which can improve diagnostic accuracy 4