You spit into a cup and then it gets sent off to the lab. It tests it to see what bugs you're growing like pseudemonas, cepacia, asprigillus, mrsa to name but a few. Then it tests to see what antiobiotics will work to fight of these bugs. If shows you're sensitive to an antibiotic it means the antibiotic works for you.
Its really important to do sputem cultres. Im suprised original poster that you have to suggest doing one to your doctor.