My mother 65 year have persistent dry cough and throat irritation from last 5 month , it's come in break someday there is no cough and someday she cough constantly we have done everything CECT scan of chest .PFT . And other blood test but everything came clean. Doctor said may be it's allergies or initial stage of asthma but he is not sure. She took asthma and allergies medicine for something and cough stopped but it start again .