Kidney renal clearance equation

From either a Cp or In Cp versus time plot, one feature is immediately clear the drug concentration drops over time. This process is called elimination and is determined by clearance (CL). Clearance is the process of removal of drug from the bloodstream. As was discussed in Chapter 3, clearance occurs primarily either through filtration of a drug by the kidneys (renal clearance, CLR) or metabolism of a drug in the liver by the action of enzymes (hepatic clearance, CLH). Other clearance processes are possible, but CLR and CLh normally comprise the large majority of total clearance (CLy or simply CL) (Equation 7.6). 

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... 

Digoxin is not extensively metabolized in humans almost two thirds is excreted unchanged by the kidneys. Its renal clearance is proportional to creatinine clearance and the half-life is 36-40 hours in patients with normal renal function. Equations and nomograms are available for adjusting digoxin dosage in patients with renal impairment. 

Assuming that the renal clearance of BSP is independent of the plasma concentration and that the plasma concentration after a standard dose falls exponentially over the period of excretion into the urine with a decay constant a, the amount of BSP (U) excreted by the kidney during the time t is given by equation... 

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. 

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