Thank you for your inquiry.
As you are well aware, the vast majority of instances of urticaria are idiopathic. From your description, the most likely diagnosis in your patient is delayed pressure urticaria. More than likely there is no valid relationship between his diet and the symptoms. I doubt that any further testing would be of any value to you, but if any further tests are performed, I would simply order a CBC, sedimentation rate, and blood chemistries.
For your interest, there is a nice review of delayed pressure urticaria in the reference (abstracted below) noted below.
Thank you again for your inquiry and we hope this response is helpful to you.
Dermatol Ther. 2009 Nov;22 Suppl 1:S22-6. doi: 10.1111/j.1529-8019.2009.01268.x.
An overview of delayed pressure urticaria with special emphasis on pathogenesis and treatment.
Cassano N, Mastrandrea V, Vestita M, Vena GA.
2nd Dermatology Clinic, Department of Internal Medicine, Immunology and Infectious Diseases, University of Bari, Bari, Italy.
Delayed pressure urticaria (DPU) is a physical urticaria characterized by the development of deep swellings at sites of pressure application on the skin. Etiopathogenesis of DPU is still unknown, although the available evidence suggests the involvement of mast cells through non-immunologic mechanisms and the role of several mediators beyond histamine, such as proinflammatory cytokines. The management of DPU is complex, also considering that prevention is very difficult and DPU frequently coexists with chronic "idiopathic" urticaria. Moreover, H1-antihistamines, which are the mainstay of treatment for common urticaria, usually provide less satisfactory results as compared with other urticarias. Therefore, numerous treatment alternatives have been proposed for severe refractory cases, such as oral or topical corticosteroids and various drugs with anti-inflammatory effects.
Phil Lieberman, M.D.