How to differentiate between Eosinophilic Granulomatous Polyangiitis and Henoch-Schönlein Purpura

We present a 29-year-old gentleman admitted with eight years of recurrent, bilateral, erythematous, macular rash involving the legs, occurring every summer. Other symptoms included intermittent joint aches with dysaesthesia. He was investigated previously with a renal biopsy, a skin biopsy and endoscopy and had been diagnosed as Henoch-Schönlein Purpura (HSP). However, his past medical history included progressive asthma and sinusitis. Investigations revealed peripheral eosinophilia, positive rheumatoid factor and elevated IgE levels with mild renal impairment. During admission, he became hypoxic. A CT pulmonary angiogram (CTPA) showed changes consistent with early pulmonary manifestations of EGPA, which improved on a repeat scan after commencing the patient on high dose prednisolone. Revising the diagnosis, his condition fulfilled criteria for ANCA negative EGPA rather than HSP.