Why is he on antibiotics? If he has an infection that seems resistant to the antibiotics he's currently on, it might be worth while to ask for a culture and sensitivity (C&S) on material collected from the site of the new infection. This test grows out the organism(s) and then tests them against various antibiotics, to determine which antibiotics would be most effective. The one potential drawback is that sometimes, when antibiotics are already being used, the bugs' growth is inhibited. Sometimes, the patient has to be taken off all antibiotics before a C&S yields useful information.