Expected (null interactive) surface as computed by double integration of time-kill data (from Fig. 1); concentration ranges: meropenem, 0 to 64 mg/liter, and tobramycin, 0 to 32 mg/liter (A), and meropenem, 0 to 5 mg/liter, and tobramycin, 0 to 4 mg/liter (B).