Which laboratory finding supports a prerenal etiology of azotemia over intrinsic renal injury?

• A. Low urine sodium concentration
• B. High urine sodium concentration
• C. Red blood cell casts
• D. Proteinuria with hyaline casts

Answer: (A)

Prerenal azotemia arises from reduced blood flow to the kidneys, so the kidneys try to hold onto sodium and water to preserve volume. This means urine sodium becomes very low because the tubules reabsorb it avidly. A urine sodium concentration that is low (commonly below 20 mEq/L) reflects this sodium retention. The kidney’s preserved reabsorptive function in prerenal states also keeps urine concentrated and FE Na (fractional excretion of sodium) low (typically under 1%).

In intrinsic renal injury, tubular damage impairs reabsorption, so sodium is lost in the urine and urine sodium rises (often above 40 mEq/L) with a higher FE Na. The pattern of casts also shifts toward findings like granular casts in acute tubular necrosis, or RBC casts with glomerular disease, rather than the prerenal pattern.

Red blood cell casts point to glomerular pathology rather than prerenal causes, and while hyaline casts can appear with concentrated urine, they are nonspecific and proteinuria with hyaline casts is not a hallmark of prerenal azotemia.