genitourinary disorders
Evaluation of the Renal Patient
Evaluating Kidney Function
Kidney function is evaluated using values calculated from formulas based on results of blood and urine tests. Glomerular filtration rate (GFR), the volume of blood filtered through the kidney per minute, is the best overall measure of kidney function; it is expressed in mL/min. Because normal GFR increases with increasing body size, a correction factor using body surface area (BSA) typically is applied. This correction is necessary to compare a patient’s GFR to normal and to define different stages of Chronic Kidney Disease . Given the mean normal BSA of 1.73 m 3 months. The standard for GFR measurement is inulin clearance. Inulin is neither absorbed nor secreted by the renal tubule and therefore it is the ideal marker for evaluation of kidney function. However, its measurement is cumbersome and, therefore, is mostly used in research settings. GFR is more accurately estimated by combining CKD-EPI equations based on creatinine and cystatin C than by equations that use creatinine or cystatin C alone ( Evaluation references ). Creatinine is produced at a constant rate by muscle metabolism and is freely filtered by the glomeruli and also is secreted by the renal tubules. Because creatinine is secreted, creatinine clearance (CrCl) overestimates GFR by about 10 to 20% in people with normal kidney function and by up to 50% in patients with advanced renal failure; thus, use of CrCl to estimate GFR in Chronic Kidney Disease is discouraged. Using a timed (usually 24-hour) urine collection, CrCl can be calculated as where UCr is urine creatinine in mg/mL, UVol is urine volume in mL/min of collection (1440 minutes for a full 24-hour collection), and PCr is plasma creatinine in mg/mL. Because serum creatinine by itself is inadequate for evaluation of kidney function, several formulas have been devised to estimate CrCl using serum creatinine and other factors. TheCockcroft-Gaultformula can be used to estimate CrCl. It uses age, lean body weight, and serum creatinine level. It is based on the premise that daily creatinine production is 28 mg/kg/day with a decrease of 0.2 mg/year of age. TheModification of Diet in Renal Disease (MDRD)study formula (current 4-factor formula) can also be used, although it requires a calculator or computer: , and may be more useful in evaluating patients with normal or near-normal kidney function. Like the Cockcroft-Gault and MDRD equations, it is also based on the serum creatinine level. Estimating GFR using the 2021 CKD-EPI creatinine equation without race is now recommended ( Evaluation references ). where SCris serum creatinine in mg/dL, kappa (κ) is 0.7 for females and 0.9 for males, α is -0.241 for females and -0.302 for males, min indicates the minimum of SCr/κ or 1, and max indicates the maximum of SCr/κ or 1. A calculator is available from theNational Kidney Foundation. 1.Inker LA, Schmid CH, Tighiouart H, et al: Estimating glomerular filtration rate from serum creatinine and cystatin C.N Engl J Med5;367(1):20-29, 2012. doi:10.1056/NEJMoa1114248 2.Delgado C, Bawega M, Burrows NR, et al: Reassessing the inclusion of race in diagnosing kidney diseases: An interim report from the NKF-ASN task force.Am J Kidney Dis78(1):103-115, 2021. doi: 10.1053/j.ajkd.2021.03.008 3.Delgado C, Bawega M, Crews DC, et al: A unifying approach for CFR estimation: Recommendations of the NKF-ASN Task Force on reassessing the inclusion of race in diagnosis kidney disease.Am J Kidney Dis79(2):268-288.e1, 2022. doi: 10.1053/j.ajkd.2021.08.003 