All of my exacerbations are a little different each time, it seems. Sometimes I am run down and feel awful just walking around the house (tired, etc.) but my numbers may not be down very far. I'd still go on IV abx in these cases cause I just feel wore out and that my body needs them. Usually, my numbers bounce above my baseline on abx.
Sometimes, my numbers are rather low and this may or may not surprise me. If numbers are low but I'm surprised by that or not coughing much out (which is the case this week), the docs look at other things as causing the problem (i.e. inflammation, GERD, etc.) before recommending abx. When the numbers are low and I feel kinda crappy, I usually would still go on abx cause I feel its just something "brewing" that may get far far worse.
Aside from regulating my blood sugars, this is one of the worst things about the disease for me. I hate not knowing when to go in, and when you're down to very limited options due to adverse reactions from medications and even the docs don't want me to go on abx unless absolutely necessary, it is very stressful.
Oh, and with regard to not bringing anything up, my doc said that is one of the most classic signs that it is abx time... an INCREASE in sputum and thickness, etc. This has also thrown me for a loop lately cause for the last few months I'm coughing very very little up, even with treatments and huffing (and laughing, which ALWAYS brought stuff up before for me). So, the docs at my current clinic told me no matter how awful I feel, if I'm not coughing a bunch up, they want to see if something else is responsible before jumping to abx.
Hope this helps.