October 2022  |  Linda Armstrong  |  ✓  Medically reviewed by: Gary Falcetano, PA-C, AE-C; Fabio Iachetti, MD; Eva Södergren, PhD, MSc 

A licensed Physician Assistant with more than 25 years of diverse experience in emergency and disaster medicine, primary care, and allergy and immunology, Gary Falcetano is the U.S. Clinical Affairs Manager for Allergy in ImmunoDiagnostics at Thermo Fisher Scientific. Fabio Iachetti is a licensed physician with more than 15 years of diverse experience in several disease areas such as allergy, CV, pain, GI, rheumatology, urology, and diabetology. He is a Senior Medical Manager for Allergy in ImmunoDiagnostics Global Medical Affairs at Thermo Fisher Scientific. A nutritionist by training, Eva Södergren now works as a Senior Scientific Advisor for Allergy on the Medical and Scientific Affairs team for Thermo Fisher Scientific’s ImmunoDiagnostics division.

Types of Blood Tests

There are two broad types of specific IgE blood tests: whole allergen tests and allergen component tests. Paired with a symptom history and physical exam, whole allergen tests help your healthcare provider to determine if you’re sensitized to a specific allergen or group of allergens.

Meanwhile, testing with allergen components helps providers identify the specific molecules (e.g., proteins, carbohydrates, enzymes) that may cause reactions. Knowing which molecules you’re sensitized to may help your provider determine if there could be an increased risk for a severe systemic reaction.

Additionally, testing with allergen components might reveal to providers that you can tolerate an allergen in a specific form. For example, if you have an egg allergy, this testing may suggest that you’re likely to tolerate baked eggs but not lightly cooked or raw eggs. (That means you may be able to eat baked goods such as sweets, breads, and more.)

Those healthcare providers who have used blood tests for decades may be most familiar with the term radioallergosorbent test (RAST), which measures specific IgE but is actually a term for an older technology. In fact, it doesn’t represent modern specific IgE blood tests for allergies, and any literature about RAST likely doesn’t apply to current tests. 

Blood Tests vs. SPTs

Specific IgE tests and SPTs are frequently used to help diagnose allergies. But how do you and your healthcare provider choose between them? Here are some key differences followed by a handy chart to further highlight the comparison. 

• Testing Personnel. While rare, SPTs can cause life-threatening anaphylaxis.¹ Thus, SPTs typically must be administered and interpreted by allergy specialists, who are trained to deal with anaphylaxis, and in a facility that is capable of providing life-saving tactics and equipment.^2-3 Meanwhile, blood tests, which carry no risk of anaphylaxis, can be ordered by almost any healthcare provider and carried out via a blood draw in a clinic or perhaps a laboratory.⁴
• Speed to Diagnosis. SPT results are often obtained in less than an hour, and blood test results are usually back from the lab within a few hours to a few days.³ However, when you consider that it may take weeks to months to secure an appointment with a specialist—and that you may need to see a general practitioner first to get a specialist referral—a blood test may actually lead to a faster diagnosis.
• Comfort. With an SPT, the provider places allergen droplets on the skin and then scratches over them to get the allergen into your skin. Or, he or she uses a sharp device to prick your skin up to 50 times with different allergens.⁴ A blood test typically requires a single needle stick for a blood draw. 
• Medications. For best results, patients must avoid most antihistamines for at least seven days prior to SPTs. Healthcare providers may also recommend stoppage of other pharmaceutical such as psychiatric medications and beta-blockers prescribed for high blood pressure.⁵ Specific IgE blood tests allow you to remain on these medications prior to testing.
• Skin Disease. For accurate SPT interpretation, you need an adequate area of normal skin. Thus, SPTs may not be an option if you have widespread skin issues such as atopic dermatitis or eczema. Meanwhile, skin condition has no effect on blood testing.⁶
• Age and Skin Reactivity. Skin reactivity can vary with age. So while it has no impact on blood testing effectiveness, SPT reliability isn’t as high in infant and elderly populations.⁶
• Skin Pigmentation. Some sources suggest that it may be more difficult to accurately interpret SPT results in those with increased skin pigmentation.⁷