Concentration (of drug) in the plasma at t=0
The concentration in the plasma at zero time (C0 or Cp0) is the theoretical concentration of a one-compartment drug in the plasma at time = 0, following instantaneous mixing throughout the compartment and before any drug has been eliminated. Cp0 is equal to the Y-axis intercept of a first-order elimination curve following bolus IV administration of a one-compartment drug. Since there is no point that can be plotted at t=0 because instantaneous mixing can not occur, the intercept is typically found by back-extrapolating the straight line obtained when data are plotted on a logarithmic Y-axis.