Allergy Blood Testing

 Radioallergosorbent testing (RAST) is an outdated form of allergy testing that involves measuring specific allergic antibodies from a blood sample. While RAST is still available, newer forms of blood testing for allergies involves the use of enzyme linked immunosorbent assays (ELISA), which involves the binding of allergic antibodies in a blood sample to an allergen, which results in a color change when a developer is added. The darkness of this color change can be measured and translated into a concentration or amount of allergic antibody in the blood sample. While the quality of allergy blood testing has improved in recent years, it is still limited in the number of tests available, as well as the smaller amount of minor allergens present in a particular test (such as a certain pollen or pet dander).

Allergy blood testing has recently become more useful in the diagnosis and management of food allergies, however. While skin testing to foods can give a sense, based on the size of the reaction, whether a person is truly allergic to the food, allergy blood testing actually measures the amount of allergic antibody to the food. This value can help determine is a child has possibly outgrown the food allergy, for example.

The high cost of allergy blood testing, as opposed to the less expensive skin test, as well as the delay in results of days to weeks, also makes it less desirable than skin testing. Skin testing also continues to be the better test, with less false positive and false negative results.

Skin testing is extremely safe, especially when performed by an allergist experienced in the diagnosis of allergies. Whole body allergic reactions, sometimes called anaphylaxis, are extremely rare from skin testing. However, given the possibility that anaphylaxis could occur as a result, skin testing should only be performed in a doctor’s office with equipment available to treat such reactions.

Young children can also be safely skin tested, including infants. Typically, infants have testing for food allergies, although they may have pet or dust mite allergies as well. Children less than 2 years of age may have less skin reactivity to allergy skin tests.

Since allergy blood testing involves testing for allergies on a person’s blood, there is no chance that the person will develop an allergic reaction as a result of the testing. However, the chance that a person will have a side effect from drawing blood, such as fainting, excessive bleeding, or infection, is actually higher than that of a side effect from allergy testing.

Certain groups of people cannot have skin testing, and therefore allergy blood testing is a better test. These groups include those who cannot stop their antihistamine medications; those with sensitive skin (and a “reaction” to the negative control), those taking certain blood pressure medications (such as beta-blockers), and those with serious heart and lung conditions that put them at increased risk if anaphylaxis should occur.

Challenging a person to an allergen means that the person is deliberately exposed to the substance, such as having the person eat a food to which an allergy is suspected. Food challenges are frequently performed to see if a child has outgrown a food allergy, or if a positive skin test actually represents an allergy. Food challenges are potentially very dangerous and should only be performed by allergy physicians experienced in their use.

Challenging a person to a non-food allergen, such as pollen or pet dander, is not usually done in an office setting; however, these tests may be performed in academic or research settings.

Want to keep learning? Watch how a skin test is performed.

Source: Practice Parameters for Allergy Diagnostic Testing. Ann Allergy Asthma Immunol. 1995; 75(6): 543-625.

DISCLAIMER: The information contained in this site is for educational purposes only, and should not be used as a substitute for personal care by a licensed physician. Please see your physician for diagnosis and treatment of any concerning symptoms or medical condition.

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