Chapter 9

Calculation of Bioavailability Parameters

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Method of Inspection

The method of Residuals and the Wagner-Nelson methods are useful technique for determining good estimates of ka. A computer program providing non-linear regression analysis may be able to provide even more accurate estimates of ka. Therefore, a quick, approximate method might be of interest. The method of inspection could be useful in this role. It is capable of providing a quick, approximate estimate of ka for checking the results obtained from other methods or as an initial estimate for more detailed analysis (Swintosky, J.V. et al., 1969).

Requirements for the Method of Inspection

The Method

The first step is to estimate the time of the peak drug concentration by inspection. If we assume that the time of peak is approximately five time the absorption half-life:


Equation 9.4.1 Time of Peak Concentration


t1/2 absorption

Equation 9.4.2 Drug Absorption Half-Life, t1/2 (absorption)

From this value for t1/2 (absorption) we can estimate the absorption rate constant.


Equation 9.4.3 Absorption rate constant

An Example

Considering the results illustrated in Figure 9.4.1 the time of peak can be estimated to be approximately 1.5 hours.

Linear plot illustrating t(peak)

Figure 9.4.1 Linear Plot of Drug Concentration versus Time after Oral Administration Illustrating tpeak

With a tpeak of 1.5 hour the t1/2 (absorption) can be estimated as 1.5/5 = 0.3 hour. And ka can be estimated as ln(2)/0.3 = 0.693/0.3 = 2.3 hr-1. For comparison the ka value used to calculated these data was 2 hr-1.


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