Here's another twist.
When they do breakpoint testing (which is what it's caled when they test suspeptibility), it's for IV concentrations.
TOBI, unlike tobramycin, can reach much higher concentrations in your lungs. Because it's not systemically available, and it goes direclty to the site of the infection, TOBI can be placed in the lungs at much higher concentrations than IV's.
So IV tobra might not work, but TOBI may. If TOBI can get to the site of the infection. Sometimes it can't because mucus is blocking the way.
Combo therapy for resistance is an option. 1 antibiotic + 1 other antibiotic = strength of 3 antibiotics, my doc explains to me. synergy testing can be done, so you're not SOL if you're aminoglycoside resistant.