Thank you for your inquiry.
It is generally accepted that a positive intradermal skin test to insect venom at a concentration of less than or equal to 1.0 mcg per ml demonstrates the presence of specific IgE antibodies. Once the presence of specific IgE antibodies is detected, in the face of a previous systemic reaction, there is an indication for venom immunotherapy.
In the Parameters to which you refer, “For those patients who have negative skin test responses despite a convincing history of anaphylaxis after an insect sting, especially if they experienced serious symptoms, such as upper airway obstruction or hypotension, it is advisable to consider in vitro testing for IgE antibodies or repeat skin testing before concluding that immunotherapy is not indicated.”
In addition, “Because patients who have a history of an allergic reaction to an insect sting and have a positive skin or in vitro test result for specific IgE antibodies to insects might be at risk for subsequent life-threatening reactions if re-stung, immunotherapy should be considered in such patients.”
Reading further in the same document, “…skin testing is the most reliable and preferred diagnostic method to identify venom-specific IgE.”
Taking these quotes from the Parameters you cited (Golden DB, et al.), “Stinging insect hypersensitivity: a practice parameter update 2011,” J Allergy Clin Immunol 2011 (April); 127(4):852-854 e23, we can answer your specific questions.
1. Either a blood test or a skin test may be used to detect the presence of specific IgE and initiate immunotherapy.
2. There is no reason to do both tests. The skin test is the test of choice, so it is done first, and is considered more sensitive. If negative, then one proceeds with the blood test. A small number of patients will be positive on in vitro testing that are negative on skin testing, but a larger number of patients are positive on skin tests than on in vitro tests.
3. Regardless of how long ago the previous systemic reaction was, if a patient exhibits specific IgE to venom, they remain a candidate for immunotherapy.
4. As mentioned above, there is no sense in trying to increase the sensitivity by doing both tests simultaneously since the vast majority of times the skin tests will be negative. There is no need to add the additional expense of doing an in vitro test.
Thank you again for your inquiry and we hope this response is helpful to you.
Phil Lieberman, M.D.