The calculation of the area under these two curves is based on a mathematical fit to the data points using a multiple exponential function. Since the data collection is terminated after 4 days these curves must then be extrapolated to infinity. A very conservative estimate is employed by assuming that the clearance following the final measured data point is based simply on the physical decay. This ignores any biological clearance and results in an overestimation of the area of these tails and hence in the radiation dose per millicurie administered.