Thank you for your query at DoctorSpring.com
The hallmark of bronchial asthma is a reversible airway obstruction, precipitated by triggers. Such a history is lacking from you..
Additionally as all examinations results are negative so I don't think it's asthma. Kindly take a ct of chest to obtain more clarity of the same. Once you have the CT report, you can upload it along with the CXR report. This will give us more clarity.
Also, a disease without wheeze or without response to salbutamol is unlikely to respond to singulair, and unlikely to be asthma. The hall mark of asthma is a pefr variability, diurnal and seasonal, which is also lacking with you. Please feel free to ask any doubt,
Dr. Jacob George P
MD IDCCM FCCP