What renal index is most consistent with prerenal azotemia secondary to dehydration in a 40-year-old male patient with impaired renal function (elevated creatinine) and a history of dehydration?

Renal Indices in Prerenal Azotemia

The urine osmolality of 450 mOsm/kg is most consistent with prerenal azotemia secondary to dehydration in this patient. 1, 2

Diagnostic Approach to Prerenal Azotemia

In prerenal azotemia from dehydration, the kidneys respond to reduced perfusion by maximally concentrating urine and avidly retaining sodium to preserve intravascular volume. 1 This creates a characteristic pattern of renal indices that distinguishes prerenal causes from intrinsic renal injury (acute tubular necrosis).

Analysis of Each Index

Urine Osmolality (450 mOsm/kg) - CORRECT:

• In prerenal azotemia, urine osmolality is typically >500 mOsm/kg, reflecting maximal urinary concentration. 2
• While this patient's value of 450 mOsm/kg is slightly below the classic threshold, it still demonstrates significant concentrating ability and is consistent with prerenal azotemia, especially given the clinical improvement with hydration. 2
• Values <350 mOsm/kg would suggest acute tubular necrosis instead. 2

Urine Sodium (18 mEq/L) - CORRECT:

• Prerenal azotemia characteristically shows urine sodium <20 mEq/L (some sources cite <10 mEq/L). 1, 2
• This reflects avid sodium retention by functioning tubules responding to volume depletion. 1
• Urine sodium >40 mEq/L would suggest acute tubular necrosis. 2

Fractional Excretion of Sodium (FENa 2.5%) - INCORRECT:

• Prerenal azotemia demonstrates FENa <1%, with 100% sensitivity for prerenal causes. 1, 2
• This patient's FENa of 2.5% is elevated and would typically suggest acute tubular necrosis rather than prerenal azotemia. 1, 2
• FENa >1% indicates impaired tubular sodium reabsorption, inconsistent with functional prerenal physiology. 2

Urine-Plasma Creatinine Ratio (19) - INCORRECT:

• Prerenal azotemia shows urine-to-plasma creatinine ratio >40. 2
• This reflects concentrated urine with high creatinine content relative to plasma. 2
• A ratio of 19 is too low and would suggest acute tubular necrosis (typically <20). 2

BUN/Creatinine Ratio (12:1) - INCORRECT:

• Prerenal azotemia typically shows BUN/creatinine ratio >20:1. 3
• The disproportionate BUN elevation occurs because urea is reabsorbed in the proximal tubule and collecting ducts during volume depletion. 4
• A ratio of 12:1 is within normal range (10-20:1) and does not support prerenal azotemia. 3

Clinical Context

The patient's clinical course—creatinine improving from 3.6 to 2.1 mg/dL after 5 days of hydration—confirms prerenal azotemia, as this condition is potentially reversible if the underlying hypoperfusion is corrected. 1 The bland urine sediment expected in prerenal azotemia (not mentioned but implied by the diagnosis) further supports this. 1

Important Caveats

• Diuretic use can falsely elevate FENa even in prerenal states, making it less reliable. 5
• In such cases, fractional excretion of urea (FEUrea) <28% may better discriminate prerenal azotemia from acute tubular necrosis, with 75% sensitivity and 83% specificity. 5
• The renal failure index (urine sodium ÷ urine-to-plasma creatinine ratio) provides additional discrimination when standard indices are equivocal. 2