Chronic Cough

Muneer,
Your mother is in India and likely she is not a smoker.
As you mentioned X Rays are normal, so are labs.
The origin of cough is usually at various sites, starting from pleura or lining of lung to lung itself, bronchial tubes, pulmonary blood vessels, trachea and bronchial tubes, throat, larynx, dripping from sinuses, uvula irritating the throat,even wax in the ear can cause cough and list goes on.
The tests should exclude dangerous diseases, TB, Cancer Lung Infections and Allergies.
Tell your Doctors to do a Chest and Neck CT including sinuses without contrast.
Check Blood IgE levels and examine Ear Nose and Throat and most likely cause will be revealed.
If not do a diagnostic bronchoscopy.
In near future I will place an article in www.thorax.us about chronic cough and its workup .
I hope this helps your mom.