Clinical Implications of Estimating Equations for Glomerular Filtration Rate

Chronic kidney disease is a major public health problem, with increasing incidence and prevalence, poor outcomes, and high costs (1, 2). In the United States, the estimated prevalence of all stages of chronic kidney disease is 20 million (1), and the number of patients receiving dialysis should exceed 650 000 by 2010 (2). Long-term adverse outcomes associated with chronic kidney disease include kidney failure, complications of impaired kidney function, and, more commonly, an increased risk for cardiovascular disease and death (3). The clinical practice guidelines of the National Kidney Foundation's Kidney Disease Quality Outcome Initiative (NKF-K/DQOI) defined chronic kidney disease as a glomerular filtration rate (GFR) less than 60 mL/min per 1.73 m² or the presence of kidney damage for 3 or more months, regardless of cause (4). The guidelines also provided a system for staging the severity of kidney disease, primarily on the basis of GFR, with stage-specific recommendations for evaluation and management (Table).

The central role of GFR in these guidelines reflects a consensus that it is the best overall measurement of kidney function and the measure most easily understood by physicians and patients. Using GFR, public health campaigns can focus on messages such as “Know your number” or “Save your GFR,” a strategy analogous to that used for hypertension or hyperlipidemia. Unlike blood pressure or serum cholesterol levels, however, we cannot measure GFR directly. Urinary clearance is cumbersome to measure in clinical practice and is often inaccurate because of incomplete urine collection. Instead, physicians use the serum creatinine level to estimate GFR. However, in addition to the GFR, the serum creatinine level depends on creatinine generation, extrarenal elimination, and tubular secretion. Using serum creatinine alone to estimate GFR is unsatisfactory and leads to delays in diagnosis and treatment of chronic kidney …