Question category: Respiratory
A 52 year old presents with normally well controlled asthma presents with, shortness of breath, wheeze and nocturnal cough. He has noticed coughing up blood on several occasions. He also complains of feeling generally unwell with a headache and fever. Bloods reveal an elevated eosinophil count and IgE. A CXR reveals new infiltrates. Which of the following will confirm the diagnosis?