In acute tubular necrosis, what is typically observed regarding urine specific gravity?

• A. High specific gravity
• B. Normal specific gravity
• C. Low specific gravity
• D. Variable specific gravity

Answer: (C)

ATN damages tubular epithelial cells, especially in the proximal tubule and thick ascending limb, so the kidney loses its ability to concentrate urine. Because water and solute reabsorption in the tubules is impaired, the urine becomes relatively dilute. This produces a low urine specific gravity, often around 1.010 (isosthenuria), reflecting a urine osmolarity close to plasma.

This helps distinguish ATN from prerenal states, where reduced renal perfusion concentrates urine and yields a high specific gravity. In the diuretic phase of ATN, urine output can rise and the urine may become more dilute or variable, but the typical acute finding is a low, nonconcentrated urine.