I was initially told a 15% difference indicates asthma, and I did not make that cut--usually got a difference of less than 5%.
Then when I went to National Jewish in Denver, I was off symbicort for a week and my difference was bigger. And when they did a methacholine challenge test--PFTs over and over with increased inhalation of an irritant--definitive asthma dx.
I think if someone hasn't recently been exposed to a trigger or is not getting over an inflammation causing virus (me now--clear mucus and tightness outta control) or takes a long-acting med like advair or symbicort, the difference pre and post albuterol in the controlled environment of the drs' office can be a false neg.