i have had a lung function test and i think its just to establish the severeity of any breathlessness.
i had mine after a chest x ray too, i find i get breathless and find it difficult to completely empty my lungs, it measures all sorts related to breathing and the worst part of it for me was to keep blowing out in order to empty your lungs, its a bit like where the police do the breath test when you have to keep blowing until they say you can stop and then they know how much you can empty your lungs if i remember correctly.
i also have asthma but last year that was doubted too as the medications for asthma gave me no relief. i think im just a complete mystery
anyway im sure it will all be fine and look forward to hearing such.