Dong Yishan, Silver Stephen M, Sterns Richard H
Rochester General Hospital, Rochester, NY, USA.
University of Rochester School of Medicine and Dentistry, Rochester, NY, USA.
Nephrol Dial Transplant. 2023 Mar 31;38(4):811-818. doi: 10.1093/ndt/gfab337.
Spot determinations of the urine creatinine concentration are widely used as a substitute for 24-h urine collections. Expressed as the amount excreted per gram of creatinine, urine concentrations in a single-voided sample are often used to estimate 24-h excretion rates of protein, sodium, potassium, calcium, magnesium, urea and uric acid. These estimates are predicated on the assumption that daily creatinine excretion equals 1 g (and that a urine creatinine concentration of 100 mg/dL reflects a 1 L 24-h urine volume). Such estimates are invalid if the serum creatinine concentration is rising or falling. In addition, because creatinine excretion is determined by muscle mass, the assumption that 24-h urine creatinine excretion equals 1 g yields a misleading estimate at the extremes of age and body size. In this review, we evaluate seven equations for the accuracy of their estimates of urine volume based on urine creatinine concentrations in actual and idealized patients. None of the equations works well in patients who are morbidly obese or in patients with markedly decreased muscle mass. In other patients, estimates based on a reformulation of the Cockroft-Gault equation are reasonably accurate. A recent study based on this relationship found a high strength of correlation between estimated and measured urine output with chronic kidney disease (CKD) studied in the African American Study of Kidney Disease (AASK) trial and for the patients studied in the CKD Optimal Management with Binders and NictomidE (COMBINE) trial. However, the equation systematically underestimated urine output in the AASK trial. Hence, an intercept was added to account for the bias in the estimated output. A more rigorous equation derived from an ambulatory Swiss population, which includes body mass index and models the non-linear accelerated decline in creatinine excretion with age, could potentially be more accurate in overweight and elderly patients. In addition to extremes of body weight and muscle mass, decreased dietary intake or reduced hepatic synthesis of creatine, a precursor of creatinine or ingestion of creatine supplements will also result in inaccurate estimates. These limitations must be appreciated to rationally use predictive equations to estimate urine volume. If the baseline urine creatinine concentration is determined in a sample of known volume, subsequent urine creatinine concentrations will reveal actual urine output as well as the change in urine output. Given the constraints of the various estimating equations, a single baseline timed collection may be a more useful strategy for monitoring urine volume than entering anthropomorphic data into a calculator.
尿肌酐浓度的即时测定被广泛用作24小时尿液收集的替代方法。以每克肌酐排泄量表示，单次排尿样本中的尿浓度常被用于估计蛋白质、钠、钾、钙、镁、尿素和尿酸的24小时排泄率。这些估计基于每日肌酐排泄量等于1克的假设（以及尿肌酐浓度为100mg/dL反映24小时尿量为1L）。如果血清肌酐浓度在上升或下降，这样的估计是无效的。此外，由于肌酐排泄量由肌肉量决定，24小时尿肌酐排泄量等于1克的假设在年龄和体型的极端情况下会产生误导性估计。在本综述中，我们评估了七个基于实际和理想化患者尿肌酐浓度来估计尿量准确性的方程。没有一个方程在病态肥胖患者或肌肉量明显减少的患者中效果良好。在其他患者中，基于Cockcroft-Gault方程重新表述的估计相当准确。最近一项基于这种关系的研究发现，在非裔美国人肾脏疾病研究（AASK）试验中研究的慢性肾脏病（CKD）患者以及在CKD使用结合剂和尼托米德优化管理（COMBINE）试验中研究的患者中，估计尿量与测量尿量之间存在高度相关性。然而，该方程在AASK试验中系统地低估了尿量。因此，添加了一个截距以校正估计尿量中的偏差。一个源自瑞士流动人群的更严格方程，该方程包括体重指数并模拟肌酐排泄随年龄的非线性加速下降，在超重和老年患者中可能更准确。除了体重和肌肉量的极端情况外，饮食摄入量减少或肌酐前体肌酸的肝脏合成减少或摄入肌酸补充剂也会导致估计不准确。必须认识到这些局限性，以便合理使用预测方程来估计尿量。如果在已知体积的样本中测定基线尿肌酐浓度，随后的尿肌酐浓度将揭示实际尿量以及尿量的变化。鉴于各种估计方程的局限性，单次基线定时收集可能是比将人体测量数据输入计算器更有用的监测尿量的策略。
