Renal secretion is mostly an active process because the transport of drugs is against a concentration gradient — the drugs become concentrated within the lumen. As such, transporter proteins must be located within the tubules and so must sources of energy, cotransporters and counter-transport molecules that play a role in drug excretion. Furthermore, saturation of these transport mechanisms and competition between a variety of drugs using them may affect the rate of renal drug elimination and thus lead to drug interactions and toxicity.
Creatinine clearance (CLcr) is an excellent endogenous estimate of glomerular filtration. However, measures of renal drug clearance using CLcr do not account for tubular secretion or reabsorption of drugs. Unfortunately, quantitative measures of the relative contributions of tubular and glomerular function are not available for most drugs and CLcr remains the only guiding factor for drug dosing in renal failure. A variety of attempts have been made to estimate CLcr in adults using serum creatinine, but the most popular Cockcroft and Gault method has been shown to be among the best ‘bedside’ predictors of true 24-h measures of CLcr. buy antibiotics online
Similar estimates have been attempted in the pediatric population where the Dechaux method has been shown to provide the best estimate for children one year of age and older. Schwartz et al has provided a method for estimating CLcr in children younger than 12 months of age. These methods are summarized in Table 1. Recently, more sophisticated methods of estimating CLcr (such as that proposed by the Modification of Diet in Renal Disease Study group) have been proposed, but these usually require advanced computations that may necessitate the use of programmable calculators or computers.
TABLE 1 Calculations for estimating creatinine clearance (CLcr)
SCr Serum creatinine. Values of SCr used in calculations are measured as mg/dL (mg %). To convert from SI, divide SCr (fjmol/L) by 88.4 to obtain values in mg/dL. This would still express CLcr in mL/min. The Cockcroft and Gault equation expressing both CLcr and SCr: CLcr (mL/s) = [(140 – age in years) x (weight in kg) /(50xSCr (ymo/L))]x (0.85 in women)