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Glomerular filtration rate (GFR) can be estimated by various methods. Ideally, a substance that is easy to analyze in blood and urine, that is not secreted or reabsorbed by the tubules and is endogenous to the body (otherwise it must be injected) should be used. In the absence of such a substance, solutes such as urea and creatinine have been used for these determinations. Unfortunately for this application, creatinine is secreted by the renal tubules resulting in overestimation of GFR and urea is reabsorbed leading to underestimation in advanced renal failure. A sensible compromise is to measure both urea and creatinine clearance and use the average as a reflection of GFR (GFR = [Ccreatinine + Curea]/2).
Another important source of error in estimating GFR is the completeness of the collection in oliguric patients and those with abnormal bladder emptying. Individuals with normal renal function empty their bladders many times per day making a 24-hour urine collection fairly reliable. Since detrussor 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 detrussor contractions (at levels > 500 ml) and high residual urine volumes (> 50 ml) further invalidating the results of the collection. In patients with low urine outputs (< 300 ml/day), a 48-hour batch urine collection is recommended. For patients with neurogenic bladders and those with abnormal post-void residuals, a catheterized collection may be necessary.
In conclusion, 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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