Kidney creatinine clearance equation

Digoxin is not extensively metabolized in humans almost two thirds is excreted unchanged by the kidneys. Its renal clearance is proportional to creatinine clearance. Equations and nomograms are available for adjusting digoxin dosage in patients with renal impairment. 

O Equations to estimate creatinine clearance that incorporate a single creatinine concentration (e.g., Cockcroft-Gault) may underestimate or overestimate kidney function depending on whether acute renal failure is worsening or resolving. 

Several equations have been developed to assess unstable kidney function. The Jelliffe equation (Table 22-1) estimates creatinine clearance by considering the change in serum creatinine over a specified time period.11 While it is more mathematically difficult to calculate, it better estimates creatinine clearance in patients with rapidly changing kidney function compared to an equation that only includes a single creatinine concentration. 

Although determination of creatinine clearance rate is a standard clinical procedure, it is difficult to carry out mainly because accurate collection of total urine output over a 24-hour period is required. It can never be certain that this requirement has been met. Since creatinine is produced continuously in muscle and is cleared by the kidney, renal failure is characterized by elevated serum creatinine levels. The degree of elevation is directly related to the degree of renal failure—if it is assumed that the production of creatinine in the muscle mass is constant and that renal function is stable. When these assumptions are valid, there is a direct relationship between serum creatinine level and kanamycin half-life, as shown in Fig. 9. The equation of the line in Fig. 9 is... 

Pigure 24-9 Relationship between GFR measured using l-iothalamate clearance and estimates of creatinine clearance (CrC ) obtained using the (A) Cockcroft and Gault equation and (B) measured creatinine clearance. (From Coresh JJoto RD, Kirk KA,Whelton PK, Massry S, Jones C, et at. Creatinine clearance os a measure of GFR in screenees for the African-American study of kidney disease and hypertension pilot study. Am J Kidney D/s 1998 32 32-42.)... 

This simplified approach ignores nonrenal routes of clearance that may be significant. If a drug is partly cleared by the kidney and partly by nonrenal clearance, the above equation should be applied to that part of the dose that is eliminated by the kidney. For example, if a drug is 50% cleared by the kidney and 50% by the liver, and the normal dosage is 200 mg/d. the corrected dosage in a patient with a creatinine clearance of 20 mL/min will be ... 

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