The gold standard for the GFR measurement is inulin clearance (3). However, if a substance in the plasma is in a stable concentration, physiologically inert, and freely filtered at the glomerulus, but is not secreted, reabsorbed, synthesized, or metabolized by the kidney, the amount of that substance filtered at the glomerulus is equal to the amount excreted in the urine (2).
It is widely accepted that kidney function is best measured as glomerular filtration rate (GFR) (1). An eGFR formula derived from pooled studies analyzing both creatinine and cystatin C, and using a biology-based mathematical approach may be advantageous. The diagnostic performance of the cystatin C derived eGFR formulae at various levels of GFR is also discussed. The review also summarizes the history, features and the feasibility of cystatin C measurements as well as the most widely used formulae for the estimation of GFR in children. We summarize the currently used methods of GFR measurement, their limitations and analytical errors. A variety of formulae based on either cystatin C or creatinine or both have been developed to estimate GFR. However, it has not enjoyed widespread use despite its significantly improved diagnostic performance in the detection of impaired GFR and its independence of body composition. In the search of a better biomarker of GFR, the small molecular weight protein cystatin C has been introduced with features more similar to that of inulin, such as constant production and no non-renal elimination. Serum creatinine does not share the properties of an ideal marker of glomerular filtration rate (GFR) like inulin, but continues to be the most widely used endogenous marker of GFR.
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