Ammonium is a major urinary buffer that is necessary for the normal excretion of the daily acid load. Its urinary rate of excretion (UNH₄) may be increased several fold in the presence of extrarenal metabolic acidosis. Therefore, measurement of UNH₄ can provide important clues about causes of metabolic acidosis. Because UNH₄ is not commonly measured in clinical laboratories, the urinary anion gap (UAG) was proposed as its surrogate about 4 decades ago, and it is still frequently used for that purpose. Several published studies strongly suggest that UAG is not a good index of UNH₄ and support the concept that direct measurement of UNH₄ is an important parameter to define in clinical nephrology. Low UNH₄ levels have recently been found to be associated with a higher risk of metabolic acidosis, loss of kidney function, and death in persons with chronic kidney disease, while surrogates like the UAG do not recapitulate this risk. In order to advance the field it is necessary for the medical community to become more familiar with UNH₄ levels in a variety of clinical settings. Herein, we review the literature, searching for available data on UNH₄ under normal and various pathological conditions, in an attempt to establish reference values to interpret UNH₄ results if and when UNH₄ measurements become available as a routine clinical test. In addition, we present original data in 2 large populations that provide further evidence that the UAG is not a good predictor of UNH₄. Measurement of urine NH₄ holds promise to aid clinicians in the care of patients, and we encourage further research to determine its best diagnostic usage.

铵是一种主要的尿液缓冲液，是日常酸负荷正常排泄所必需的。在肾外代谢性酸中毒的情况下，其尿排泄率 (UNH ₄ ) 可能会增加几倍。因此，测量 UNH ₄可以提供有关代谢性酸中毒原因的重要线索。由于 UNH ₄在临床实验室中不常测量，尿阴离子间隙 (UAG) 在大约 4 年前被提议作为其替代指标，并且它仍然经常用于该目的。几项已发表的研究强烈表明 UAG 不是 UNH ₄的良好指标，并支持直接测量 UNH ₄是临床肾脏病学中定义的重要参数的概念。低 UNH_{最近发现4}水平与慢性肾病患者代谢性酸中毒、肾功能丧失和死亡的高风险相关，而像 UAG 这样的替代物并没有概括这种风险。为了推进该领域，医学界有必要更加熟悉各种临床环境中的UNH _4水平。在此，我们回顾文献，搜索正常和各种病理条件下 UNH ₄的可用数据，试图建立参考值来解释 UNH ₄结果，如果 UNH ₄测量成为常规临床测试。此外，我们提供了 2 个大型人群的原始数据，这些数据进一步证明 UAG 不是 UNH ₄的良好预测因子。尿液 NH ₄的测量有望帮助临床医生护理患者，我们鼓励进一步研究以确定其最佳诊断用途。