Measurement of RRF
To date, the glomerular filtration rate (GFR) is considered to be the best measure of overall residual renal function (RRF) and can be estimated by various methods. Ideally, GFR measurement should be based on a molecule that would be generated at a constant rate in the body, be freely filtered, not secreted or reabsorbed by the tubules and have no extra-renal clearance (1). Urinary inulin clearance is the gold standard for measuring GFR as it is an inert polyfructose molecule that exhibits many of these characteristics. However, inulin clearance is not routinely performed because it is costly, time consuming and involves infusing an exogenous substance into the blood (2,3). In the absence of a truly ideal substance, solutes such as urea and creatinine have been used for these determinations. Creatinine is secreted by the renal tubules resulting in overestimation of GFR and urea is reabsorbed passively in the proximal tubule leading to underestimation of GFR in advanced renal failure. A reasonable compromise is to measure both urea and creatinine clearance and use the average as a reflection of GFR (GFR = [Ccreatinine + Curea]/2) (4). The European Best Practice Guidelines recommend this as the preferred method for calculating GFR in advanced renal failure (5).
An important source of error in estimating GFR is the completeness of urine collection, especially in oliguric patients and those with abnormal bladder emptying (1). Individuals with normal renal function empty their bladders many times per day making a 24-hour urine collection fairly reliable. Since detrusor contractions (urge to void) generally occur when the bladder is distended to 250-350 mL, patients with advanced renal failure and low urine outputs are prone to miss the only specimen during a 24- hour urine collection or may include two samples during that period. A significant proportion of patients also suffer from neurogenic bladders as a consequence of diabetes and have abnormal detrusor contractions (at levels > 500 mL) and high residual urine volumes (> 50 mL) (6) further invalidating the results of the collection. In patients with low urine outputs (< 100 mL/day), a 48-hour batch urine collection is recommended (7). For patients with neurogenic bladders and those with abnormal post-void residuals, a catheterized collection may be necessary.
Periodic measurements of RRF are important in deciding when to initiate dialysis, in the determination of an adequate dose of PD and in evaluating clinical outcome. Proper methodology with special attention to diseases that may alter bladder function is imperative. The information obtained from these studies should be routinely incorporated in the decision process used to adjust dialysis dose.
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Smith HW: Measurement of the Filtration Rate. The Kidney: Structure and Function in Health and Disease. New York: Oxford University Press, 1951:3962
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Dombros N, Dratwa M, Feriani M, Gokal R, Heimbürger O, Krediet R, Plum J, Rodrigues A, Selgas R, Struijk D, Verger C; EBPG Expert Group on Peritoneal Dialysis. European best practice guidelines for peritoneal dialysis. 2 The initiation of dialysis. Nephrol Dial Transplant. 2005 Dec;20 Suppl 9:ix3-ix7 http://www.ncbi.nlm.nih.gov/pubmed/16263750
Daneshgari F, Liu G, Birder L, Hanna-Mitchell AT, Chacko S. Diabetic bladder dysfunction: current translational knowledge. J Urol. 2009 Dec;182(6 Suppl):S18-26 http://www.ncbi.nlm.nih.gov/pubmed/19846137
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