Formula estimation of glomerular filtration rate: have we gone wrong?

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Abstract
Estimating glomerular filtration rateThe best known function of the kidneys is plasma filtration—measured by the glomerular filtration rate (GFR). Many of the kidney's functions are related to GFR (box 1). Inulin clearance and modern isotopic methods are not practical for measuring GFR in routine practice. Creatinine based tests are used instead but have several disadvantages. Creatinine clearance involves timed urine collection and is prone to error. Measuring serum creatinine is easier but this test cannot detect early kidney disease.2 Routine reporting of estimated GFR using formulas based on serum creatinine concentration plus age, sex, and racial group was first advocated in the US and has now been recommended in many other countries.Box 1 Functions of the kidneys related to glomerular filtration rate3 Excretion of nitrogenous waste, sodium, free water, potassium, phosphate, and water soluble medicines (such as digoxin and gentamicin) Control of blood pressure Acid-base balance Secretion of erythropoietin Hydroxylation of vitamin D1 (activation) Gluconeogenesis in the fasting state Catabolism of peptide hormones (including insulin) In the UK the second part of the national service framework for renal services,4 published in 2005, required clinical biochemistry laboratories to develop automatic reporting of formula based GFR estimates. In 2006 the quality and outcomes framework5 asked primary care to establish registers of patients with estimated GFR worse than 60 ml/min/1.73 m2 (chronic kidney disease stages 3-5 in the international classification; table⇓).6 7 Many registers have been populated using computer programs that find serum creatinine results in general practitioners' information systems and then calculate estimated GFR using the “four variable version of the modification of diet in renal disease” (MDRD) formula (box 2).8Box 2 Estimating glomerular filtration rate (GFR) using the four variable version of the modification of diet in renal disease equation GFR (ml/min/1.73 m2)=186×{[Scr/88.4]−1.154} ×age in years−0.203 ×0.742 if female ×1.21 if African American Where Scr is serum creatinine in µmol/l View this table: In this window In a new window Classification of chronic kidney diseaseLabelling patients as having chronic kidney disease in this way has caused controversy.2 We challenge some of the applications of GFR estimated from this formula.

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