Effect of Renal Function on Drug Excretion

A male patient is to receive Drug9 and it is known that the elimination of this drug depends on renal function. This patient is 70 kg (154 pound), 178.3 cm (70.2 inches) height and 39 years old. The measured serum creatinine was 1.38 mg/dl. Calculate the creatinine clearance for this patient using the Cockcroft and Gault equation. (Note: Use ideal body weight if less than actual body weight). Once you have the calculated creatinine clearance for this patient estimate the expected elimination rate constant for Drug9 in this patient.

A previous study with Drug9 resulted in the following data.

 t1/2 (hr) CreatinineClearance (ml/min) 2.86 10 1.81 50 1.1 120

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Some Equations:

Equation 27.1.1 Elimination rate constant calculated from creatinine clearance

Equation 27.1.2 Creatinine Clearance calculated from a timed urine collection

UCr is the concentration of creatinine in urine in mg/dl, Vu is the volume of urine in ml, SCr is the serum creatinine concentration in mg/dl and t is the collection time in minutes.

Equation 27.1.3 Cockcroft and Gault equation for estimating Creatinine Clearance

Weight in kg and SCr is the serum creatinine concentration in mg/dl. Note, you should only multiply by 0.85 in the case of female patients. The original authors of this equation used actual body weight in Equation 27.1.3. More recently it has been recommended that ideal body (IBW) be used in this equation unless the actual body weight (ABW) is less. (Murphy, 2001, p4).

IBW (male) (kg) = 50 + 2.3 x (Ht(in) - 60) or IBW (female) (kg) = 45.5 + 2.3 x (Ht(in) - 60) (Murphy, 2001, p4-5)

References
• Ritschel, W.A. 1980 Handbook of Basic Pharmacokinetics, 2nd ed., Drug Intelligence
• Murphy, J.E. 2001 Clinical Pharmacokinetics, 2nd ed., ASHP, Bethesda, MD
• Sarubbi, F.A. and Hull, H.H. 1978. Amikacin serum concentrations: prediction of levels and dosage guidelines, Ann. Intern. Med., 89, p 612-618

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