1.0 mL serum (0.2 mL minimum) from a Non-Barrier Red Top. (Codes below) Collect trough just before next dose; 30-45 minutes after IV dose or 1-2 hours after IM dose for peak. Please indicate on requisition if patient is taking penicillin or cephalosporins. After 30 minutes of clotting, promptly centrifuge, separate and refrigerate serum within 2 hours.