In vitro allergy tests allow measurement of the amount of specific immunoglobulin E to individual allergens in a sample of blood. The amount of specific immunoglobulin E produced to a particular allergen approximately correlates with the allergic sensitivity to that substance. These tests allow determination of specific immunoglobulin E to numerous different allergens from one blood sample, but the sensitivity and specificity are not always as good as accurate skin testing.
This is a measurement of the total level of immunoglobulin E in the blood (regardless of specificity). Although patients with allergic rhinitis are more likely than the normal population to have an elevated total immunoglobulin E level, this test is neither sensitive nor specific for allergic rhinitis. As many as 50% of patients with allergic rhinitis have normal levels of total immunoglobulin E, whereas 20% of unaffected individuals can have elevated total immunoglobulin E levels. Therefore, this test is generally not used alone to establish the diagnosis of allergic rhinitis, but the results can be helpful in some cases when combined with other factors.
As with the total serum IgE level, an elevated eosinophil count supports the diagnosis of allergic rhinitis, but it is neither sensitive nor specific for the diagnosis. The results can sometimes be helpful when combined with other factors.
For more on the workup of allergic rhinitis, read here.
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Cite this: Michael A. Kaliner. Fast Five Quiz: Test Your Knowledge About Fall Allergies - Medscape - Jul 06, 2020.