This patient's presentation is suggestive of urticarial vasculitis, and a skin biopsy is indicated. He has an urticarial eruption with joint pain and low-grade fevers suggesting systemic inflammation. Typical urticarial wheals last for less than 24 hours (with individual lesions lasting only a few hours), resolve without cutaneous sequelae, and are pruritic and not painful. In contrast, this patient has painful wheals that last for more than 24 hours and leave bruise-like changes when they resolve, which is atypical for usual urticaria, and is characteristic of urticarial vasculitis. Patients with urticarial vasculitis may have an underlying autoimmune disease, most often lupus erythematosus, but even in the absence of an associated autoimmune disease, patients with urticarial vasculitis (particularly hypocomplementemic urticarial vasculitis) are at risk of multisystem disease, including nephritis. A skin biopsy is important in making the diagnosis, and guiding further work-up and treatment.

A gluten-free diet is unlikely to have any impact on urticarial vasculitis, and only rarely will gluten intake impact urticarial skin conditions. Gluten avoidance is important in patients with dermatitis herpetiformis, which is often associated with insensitivity to gluten.

Radioallergosorbent testing (RAST) may be appropriate in patients with a typical urticarial eruption from a likely food or environmental trigger; however, this patient is presenting with atypical urticarial lesions with signs and symptoms concerning for urticarial vasculitis. RAST plays no role in the diagnosis of urticarial vasculitis.

Thyroid function testing is indicated in the evaluation of chronic urticaria in which patients have regular episodes of typical urticaria over a period of more than 6 weeks. Because of the atypical presentation of this patient's wheals and the lack of clinical findings suggestive of thyroid disease, testing would not be the most appropriate next step.