Thank you for your inquiry.
Apart from lay entries and anecdotes, there is a dearth of reports of allergy to either Stevia or sucralose (Splenda). I was only able to find one report on Stevia (see reference copied below), and none for sucralose. Stevia, as you know, is taken from a plant, and therefore it would not be unreasonable to suspect that IgE could be synthesized to this substance. However, sucralose is identical to sucrose except for the substitution of three chlorine atoms for hydroxyl groups. Thus, at least potentially, one would think that an IgE-mediated reaction to this substance would be rare.
Since both substantives are soluble in water, I believe one could do skin tests to solutions of both products and conceivably could do oral challenges by adding them to a potential taste masker such as peanut butter, although I have not personally had any experience in doing so.
Thank you again for your inquiry and we hope this response is helpful to you.
Anaphylaxis by stevioside in infants with atopic eczema
Volume 62, Issue 5, pages 565–566, May 2007.
Phil Lieberman, M.D.