Estimation of function: As so many drugs depend on renal elimination, it is important to evaluate the degree of renal impairment in medication dosing. Dosing is not simply creating medication lists, but incorporating key elements of the drug's unique pharmacology and pharmacokinetics.
Renal impairment is not an “all-or-none” phenomenon, and must be evaluated in relative terms. The most common method use to estimate renal function is to calculate a creatinine clearance. The Cockcroft-Gault method is typically employed in a clinical setting.
It should be recognized that this is a limited reflection of GFR, which is an important point. Because creatinine production is based on muscle mass/turnover, and these calculations assume accurate assay of steady-state creatinine levels in the serum, they are prone to error. For example, an individual with poor muscle mass may have a low serum creatinine even in the face of diminished GFR (this often occurs in frail, elderly patients). Likewise, in acute real failure, serum creatinine is in the process of rising to a new plateau, and calculations may over-estimate the true GFR. Therefore, calculation of an estimated creatinine clearance is only one aspect of the overall evaluation. Other methods such as MDRD are available which correlate more accurately with GFR, but unfortunately most dosing guidelines have been constructed around the creatinine clearance.
Renal impairment is not an “all-or-none” phenomenon, and must be evaluated in relative terms. The most common method use to estimate renal function is to calculate a creatinine clearance. The Cockcroft-Gault method is typically employed in a clinical setting.
It should be recognized that this is a limited reflection of GFR, which is an important point. Because creatinine production is based on muscle mass/turnover, and these calculations assume accurate assay of steady-state creatinine levels in the serum, they are prone to error. For example, an individual with poor muscle mass may have a low serum creatinine even in the face of diminished GFR (this often occurs in frail, elderly patients). Likewise, in acute real failure, serum creatinine is in the process of rising to a new plateau, and calculations may over-estimate the true GFR. Therefore, calculation of an estimated creatinine clearance is only one aspect of the overall evaluation. Other methods such as MDRD are available which correlate more accurately with GFR, but unfortunately most dosing guidelines have been constructed around the creatinine clearance.
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