In cachectic patients with low creatinine production, S Cr may be “normal” with a GFR as low as 30 mL/minute.Ĭreatinine clearance = ) × Weight × 1.73 m 2 × 0.85* ] 72 × S Cr × Body Surface area * Moreover, S Cr does not increase above normal limits until GFR has decreased below 50 mL/minute. Thus, a “trivial” increase in S Cr from 0.6 to 1.2 mg/dL implies a 50% decrease in GFR. That is, a doubling of S Cr implies a halving of GFR. However, the relationship between S Cr and GFR is not direct it is exponentially inverse. Therefore, S Cr is a reliable surrogate for GFR. These come into equilibrium when renal function is in a steady state. S Cr reflects the balance between creatinine production and excretion. Urea depends on tubular excretion but may be a misleading surrogate for tubular function because its blood level is affected by nonrenal pathology, such as gastrointestinal hemorrhage and protein catabolism (abnormally increased) or malnutrition and end-stage liver disease (abnormally decreased). Blood urea nitrogen and S Cr are the most commonly used indicators of renal function.
When ARF does ensue, loss of renal solute clearance begins to result in the buildup of serum concentrations of electrolytes, urea, water, and other osmotic elements (azotemia). In summary, oliguria is common but seldom implies ARF, but the presence of a normal urine flow rate does not exclude it. In contrast, when postoperative ATN occurs, it is often a culmination of multiple lesser insults in a protected milieu, resulting in nonoliguric renal failure, defined as ARF with urine flow 15 to 80 mL/hour. With two very important exceptions (sepsis and liver failure), it is reversed by restoration of normal renal hemodynamics.
It reflects absolute or relative hypovolemia, with vasoconstriction and sodium retention as a consequence of activation of the sympathoadrenal, renin-angiotensin-aldosterone, and antidiuretic hormone systems. Perioperative oliguria is an unreliable index of renal function because it is almost inevitably prerenal in nature. Sladen, in Complications in Anesthesia (Second Edition), 2007 Recognition The impact of these variations in tubular creatinine secretion can be largely overcome by administering cimetidine, which inhibits tubular creatinine secretion, an hour before urine collection. Normally, renal tubular secretion is so minor that it does not significantly impact the creatinine clearance calculations at normal GFRs, but can create significant distortion as the real GFR drops. This is due to the fact that renal tubules secrete a small amount of creatinine into the urine (in addition to that which is filtered). However, this can be markedly simplified by forgoing the 24-hour convention for a more practical short-term (e.g., 6 hours or even 2 hours) collection, which has been shown to be just as accurate as the 24-hour assay and provides a more responsive measure of real-time function.Ī significant disadvantage of creatinine clearance is that it becomes less accurate as GFR drops. Creatinine clearance measurement does involve the inconvenience of sustained urine collection, traditionally over a 24-hour period. Fagan, in Total Burn Care (Fifth Edition), 2018 Creatinine ClearanceĬreatinine clearance is an inexpensive, consistent, time-tested technique for providing a rough estimate of GFR and renal function.