The prevalence of allergies is on the rise, with no obvious end in sight. Subsequently, the demand for allergy testing is higher than it’s ever been. But what are the different options that we can test for different allergies?
As part of our allergy testing series, in this article, we will focus on IgE testing, also known as RAST. We will cover what it is, why it’s done, how it’s done and what the results mean.
What is IgE?
Immunoglobulin E (IgE) is a type of antibody naturally found in the human body. Antibodies are proteins that the immune system makes to help fight bacteria, viruses, etc. The rise of these IgE antibodies play a significant role in triggering an allergic reaction.
When you are exposed to an allergen, the immune system starts producing IgE antibodies.
In the chain reactions that follow, IgE antibodies can go on to trigger the release of chemicals like histamine, which cause symptoms like itching, swelling, nasal congestion or even airway problems, if severe enough.
Why is IgE testing done?
IgE or RAST testing is one of the ways that we can test for allergy to a specific trigger. This test can be useful for diagnosing conditions like hayfever, dust mite allergies, specific food allergies and other specific proteins.
Types of IgE Testing
Total IgE
This is a total count of the level of IgE immunoglobulins present in the blood at any one time. Elevated levels may indicate that there is an allergic reaction going on, but are not specific enough to identify which allergens may be causing this reaction.
Total IgE levels may also be elevated in atopic eczema, asthma, parasitic infections and allergic bronchopulmonary aspergillosis.
Specific IgE
When your GP says they can do an allergy blood test, this is what they would usually refer to.
They would take a blood sample, and then send it to a special laboratory that can test for the IgE levels to specific proteins. These proteins correspond to the allergens that you may be worried about. They can include various foods, pollen, molds, animal dander, dust mites and more.
Say for example that you had lip swelling and a rash after eating a fruit salad containing strawberries, apples, peaches, and oranges. We could then do a RAST testing and test for specific IgE antibodies in each of these four fruit. If one of them is high and out of the normal range, then this needs further investigation.
IgE antibody levels can be tested to over 1,000 different allergens. The assays exist, but the availability may differ depending on your local area. Some tests may also be more expensive than others, especially if they have to be sent to a laboratory out of area.
Is IgE/ RAST testing safe?
Yes, this is a safe test as it involves a simple blood test.
A phlebotomist at your local practice will take the blood test and ensure it arrives to the right lab for testing.
What are the different IgE levels and what do they mean?
Here is a table of the different approximate ranges of IgE levels. Please note that these may vary slightly depending on the lab and the local guidelines.
The RAST score or the IgE level grade is ranked on a scale from 0 to 6.
| Grade | Serum Concentration of Specific IgE (KUA/L) | Result Description | Notes |
|---|---|---|---|
| 0 | <0.35 | Negative | No significant level of specific IgE detected. Less likely to be an allergy-related condition but NOT ruled out. |
| 1 | 0.35-0.7 | Weak Positive | Low levels of specific IgE detected. Further investigation may be needed. |
| 2 | 0.7-3.5 | Positive | Moderate levels of specific IgE indicate an allergy to the tested substance. |
| 3 | 3.5-17.5 | Positive | Significant levels of specific IgE; allergic condition is likely. |
| 4 | 17.5-52.5 | Strong Positive | High levels of specific IgE; strongly indicative of an allergy. |
| 5 | 52.5-100 | Strong Positive | Very high levels of specific IgE; severe allergic condition is highly likely. |
| 6 | >100 | Strong Positive | Extremely high levels of specific IgE; severe allergic condition is highly likely. |
Conclusion
In conclusion, RAST or IgE testing is an accessible blood test that can act as the first port of call for someone concerned regarding an allergic reaction. A negative RAST or IgE testing however does not mean that you are not allergic to the tested substances, due to poor sensitivity. Certain medications can also interfere with the results and lead to false negatives.
Dr Elena Salagean is a consultant allergist who offers allergy testing, management and treatment options in the UK. She has been quoted by numerous journalists in the UK and USA and is also a researcher and published author on Pubmed.