A patient has been on antibiotic therapy for many days without improvement. What is the best next step to guide antibiotic selection?

Culture and sensitivity (C&S) testing is the gold standard for identifying the causative organism and its antibiotic susceptibilities. When empiric antibiotic therapy fails, C&S guides targeted therapy to ensure the correct antibiotic is selected. This is also the answer to 'how do you know which antibiotic to prescribe' — you culture the organism and test sensitivity. This avoids antibiotic resistance and treatment failure.