The probability of resistance is higher in those patients who relapse and every effort must be made to obtain a specimen that can be cultured for sensitivities. That said, most patients who relapse do so with a fully sensitive strain and it is possible that these patients have not relapsed, but have instead been re-infected; these patients can be re-treated with the same regimen as before (no drugs need to be added to the regimen and the duration need not be any longer).