Thank you for your inquiry.
I am in agreement with you in that more than likely the history of exposure to grass is probably a "red herring." However, since you presented the problem as related to grass, and it is so easy to see if there is an IgE-mediated sensitivity, I would at least test him to this one allergen. As noted, my assumption is that the test will be negative. However, this is indicated if for no other reason than to "clear the air" for the patient.
There was one episode related to exercise, and it is unclear from your email as to the extent of exertion (we know he was at least walking) related to the other events. So exercise induced urticarial/anaphylaxis must be kept in mind.
The other thing that is important to keep in mind is that the vast majority of episodes of acute urticaria/anaphylaxis are idiopathic in adults and adolescents who have no history of atopy (1).
You have done more than enough laboratory work, and I do not feel any other blood work would be of help to you. I think if you do not find a positive test to grass, you will be forced to simply make a diagnosis of recurrent idiopathic urticaria/anaphylaxis. If this is the case, of course the most important element of his treatment would be to maintain an automatic epinephrine injector and use it appropriately. You could also consider pretreatment with antihistamines if episodes began to occur more frequently.
The only other laboratory test that I might consider if the episodes continued would be a repeat tryptase. If it was above 11 you would consider doing a bone marrow. If it was 20 or above, a bone marrow would clearly be indicated.
In summary, my best guess is that your patient is experiencing what we would normally call "recurrent idiopathic urticaria/anaphylaxis" and no cause will become evident. No further workup except perhaps skin testing him to grass, in my opinion, is indicated at this time. If episodes worsen or become more frequent, you could consider repeating a serum tryptase and also pretreatment with antihistamines.
Thank you again for your inquiry and we hope this response is helpful to you.
1. Webb L and Lieberman P. Anaphylaxis: A Review of 601 Cases. Annals of Allergy, Asthma, and Immunology, July 2006; 97(1):39-43.
Phil Lieberman, M.D.