Example : A 65 years old, 43 kg man was admitted to the hospital because of acute urinary tract infection. Urine was collected over 24-hr period to determine the creatinine clearance. The total urine volume collected was 1600 ml and the creatinine conc in urine was 0.5 mg/ml. Serum creatinine was 1.5 mg/dl. Estimate the patient creatinine clearance and gentamicin half life based on the patient's kidney function.
Calculation of the creatinine clearance:
a- Based on the creatinine excretion rate:
Total amount of creatinine excreted = 1600 ml x 0.5 mg/ml = 800 mg
Exc.Rate =
800 mg
24 hr.60 min/hr
= 0.556 mg/min
CLCr =
0.556 mg/min
1.5 mg/100 ml
= 37 ml/min
Kid.Function =
37 ml/min
120 ml/min
= 0.31 of normal
b- Based on age, wt and serum creatinine:
CLCr =
(140 - age in years)(wt in kg)
72 (serum Cr mg/dl)
CLCr =
(140 - 65)(43 kg)
72 (1.5 mg/dl)
= 30 ml/min
Kid.Function =
30 ml/min
120 ml/min
= 0.25 of normal
Estimated Half life (a) =
2.5 hr
0.31
= 8 hr
Estimated Half life (b) =
2.5 hr
0.25
= 10 hr
Based on the patient's kidney function, and gentamicin half life in a patient population with similar kidney function, the half life in this patient is approximately 8-10 hr
