What is a common characteristic of urine in postrenal failure?

In postrenal failure, the characteristic of variable urine sodium is primarily due to the obstruction in the urinary tract, which affects the kidneys' ability to concentrate or dilute urine. When there is a blockage, such as from a kidney stone or tumor, the kidneys cannot effectively regulate sodium and water balance, leading to fluctuations in sodium levels in the urine.

In contrast, high proteinuria is more indicative of glomerular damage rather than postrenal causes. High specific gravity usually points towards concentrated urine often seen in pre-renal causes, as the kidneys try to retain water in response to low perfusion. Consistent high glucose levels in urine are generally associated with diabetes, and while glucose may be present in urine under certain conditions, it is not a defining characteristic of postrenal failure. Therefore, the variability in urine sodium best reflects the effects of urinary obstruction and the renal compensation mechanisms involved in postrenal failure.