The time courses of drug concentration in serum after i.v. drip infusion of 2 mg/kg of tobramycin and 25 mg/kg of cefazolin in children were analyzed by model-independent moment analysis. The volume of distribution at the steady state per body weight (Vdss/BW) of tobramycin was in the range of 212 to 335 ml/kg and that of cefazolin was 119 to 156 ml/kg. A plot of the differences of Vdss/BW obtained in the same child for tobramycin and cefazolin against the value of Vdss/BW of tobramycin gave a linear regression line (r = 0.971). The magnitude of Vdss/ (1) of tobramycin could be well interpreted as corresponding to the extracellular water volume. In the case of cefazolin, the extracellular water space accounts for about 60% of the total distribution volume. The remaining 40% of the total Vdss/ of cefazolin was considered to be accounted for by the disposing organs.
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