Evaluating urine color (dark vs light) is quick and easy, and may be an indication of the kidney’s ability to concentrate urine, which is a very sensitive indicator of renal tubular function. A more precise way to evaluate the concentration of urine is to measure urine sodium, urine specific gravity, and urine osmolality. Various primary or secondary renal problems can affect the way the kidney concentrates urine. For example, in hypovolemic states, the kidney attempts to retain water by reabsorbing sodium, causing an osmolar gradient for water to follow. The resulting urine will have low sodium, high specific gravity, and high osmolality (Table 175-2). However, in acute tubular necrosis (ATN) when renal tubules are damaged and become necrotic and dysfunctional, the kidney loses its ability to concentrate urine, and specific gravity will be identical to the specific gravity of the glomerular filtrate, which is 1.010. Assessing the ratio of urine osmolality to serum osmolality can also be helpful (Table 175-2).