Chapter 03 - Assessment of Kidney Function in Acute and Chronic Settings

Abstract

Glomerular filtration rate (GFR) is generally considered the best overall assessment of kidney function. The normal level for GFR is 120 to 130 mL/min/1.73 m2 but varies according to age, sex, body size, and other factors. Reductions in GFR can be due to either a decline in the nephron number or a decline in the average single nephron GFR, resulting from either physiologic or hemodynamic alterations. GFR is measured with plasma or urinary clearance of filtration markers. Because of the difficulties in measuring GFR, GFR is often estimated with serum levels of endogenous filtration markers. The main limitation is that the serum level of filtration markers is also influenced by generation, tubular secretion and reabsorption, and extrarenal elimination (“non-GFR determinants”) of these markers. Estimating equations incorporate demographic and clinical variables as surrogates for the non-GFR determinants and provide a more accurate estimate of GFR than the serum level alone. Serum creatinine, the most commonly used endogenous filtration marker in clinical practice, is affected by diet and muscle mass. Serum cystatin C is a useful alternative that is less affected by diet and muscle mass. In the past, urea was widely used. The KDIGO CKD 2013 clinical practice guidelines recommend eGFRcr using the Chronic Kidney Disease Epidemiology (CKD-EPI) 2009 equation as the primary test with eGFRcr-cys or eGFRcys using the CKD-EPI 2012 equations or a clearance measurement as confirmatory tests. Newly developed 2021 CKD-EPI equations that do not include a race coefficient were recommended for use in 2021 by the American Society of Nephrology and the National Kidney Foundation.