Example: After administration of a single oral dose of an antibiotic, the following data were obtained:

Time
(hr)
Conc
(mg/L)
Time
(hr)
Conc
(mg/L)
0.2 88 2.5 311
0.5 185 4.0 246
1.0 277 6.0 161
2.0 321 8.0 94

Using the method of residuals calculate the absorption rate constant.

Plot the concentration versus time on a semilog scale.

    • Time
    • Residuals
    • 0.2
    • (600 - 88)= 512
    • 0.5
    • (560 - 185)= 375
    • 1.0
    • (420 - 277)= 143
slope = -ka/2.303

Draw the line that represents drug elimination and back extrapolate to the y-axis.

Calculate the residuals (the differences between drug conc and the extrapolated line) during the absorption phase.

Plot the residuals versus time, and draw the best line that goes through the points.

The slope of the line = - ka/2.303

Slope = - 0.43 hr-1

ka = 2.303 x 0.43 = 0.99 hr-1

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