The initial plasma concentration (Cp0) after drug administration has to be estimated, since it is not possible to measure the plasma drug concentration at time zero.
a- An iv dose of the drug is administered, then plasma samples are obtained.
b- Draw the best line that goes through the sample concentrations, and back extrapolate the line to time zero.
c- The y-intercept is the plasma drug concentration at time zero (Cp0)
Cp0
The volume of distribution depends on the weight of the patient
Theophylline volume of distribution can be:
30 liters in a 60-kg woman
10 liters in a 20-kg child
50 liters in a 100-kg man
Despite the variability in the total Vd, all these patients have Vd of 0.5 L/kg
Vd has units of volume (e.g. liters)
Vd can be normalized for weight and will have units of volume/weight (e.g. liters/kg)
