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The i1/2 (elimination) is the time at which the elimination line crosses A/2 = 4.5 hour:

el t1/2 (elimination) 0 5 / The t1/2 (absorption) is the time at which the residuals line crosses A/2 = 0.7 hour:

Assuming that the 100-mg dose of drug was completely absorbed, the Vd can be calculated from equation (39):

This method of calculation is often referred to as the "method of residuals" or "feathering the curve." It is important to remember that the following assumptions were made:

1. It is assumed that ka is at least five times larger than kel; if not, neither constant can be determined accurately.

2. It is assumed that the absorption and elimination processes are both strictly first order; if not, the residuals line and, perhaps, the elimination line will not be straight.

3. It is assumed that absorption is complete; if not, the estimate of Vd will be erroneously high.

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