Suggest Test To Determine The Reason For Allergic Cough

Hi My wife is suffering from cough. Her cough is alergic cough but we are finding it difficult to know eact allergy. She has carrieed all test like ENT related, chest xray, TB test etc and all is normal. Occassionaly every after 2-3 months she suffers from heavy cough (some times dry , some times wet) and the cough lasts for 20-30 days even after taking heavy antibiotics. Can you help me in this