Example: After an iv administration of 1000 mg cefazoline urine samples were collected and the following data were obtained.
| Time interval (hr) |
volume (ml) |
Conc (mg/ml) |
| 0-1 |
65 |
5.1 |
| 1-3 |
114 |
3.0 |
| 3-5 |
140 |
1.0 |
| 5-8 |
225 |
0.3 |
| 8-12 |
180 |
0.1 |
Calculate k and ke for cefazoline
| Time interval (hr) |
volume (ml) |
Conc (mg/ml) |
Amount mg |
Exc rate (mg/hr) |
| 0-1 |
65 |
5.1 |
331.5 |
331.5 |
| 1-3 |
114 |
3.0 |
342 |
171 |
| 3-5 |
140 |
1.0 |
140 |
70 |
| 5-8 |
225 |
0.3 |
67.5 |
22.5 |
| 8-12 |
180 |
0.1 |
18 |
4.5 |
The first step is to calculate the renal excretion rate during each urine collection interval
a- Calculate the amount of cefazoline excreted during each interval by multiplying the volume by the drug concentration for each sample.
b- Calculate the renal excretion rate by dividing the amount excreted during each interval by the length of the interval.
c- Plot the renal excretion rate versus the middle time point of each urine collection interval
| Time (hr) |
Exc.rate (mg/hr) |
| 0.5 |
331.5 |
| 2.0 |
171 |
| 4.0 |
70 |
| 6.5 |
22.5 |
| 10.0 |
4.5 |
Slope = -k/2.303
half life
The y-intercept = ke Dose
Graphically the half life = 1.5 hr
k = 0.462 hr-1
Y-intercept = 400mg/hr = ke
Since the dose = 1000mg
ke = 0.4 hr-1
