Prerenal Azotemia (Volume Depletion)
The combination of markedly elevated BUN, mildly elevated serum creatinine (resulting in a BUN:Cr ratio >20:1), and elevated hematocrit in a 50-year-old woman indicates prerenal azotemia due to volume depletion (dehydration). 1, 2, 3
Pathophysiology and Diagnostic Rationale
The disproportionate elevation of BUN relative to creatinine is the hallmark finding here:
Normal BUN:Cr ratio is 10-15:1; ratios >20:1 strongly suggest prerenal azotemia, where renal hypoperfusion leads to enhanced urea reabsorption in the proximal tubule while creatinine reabsorption remains relatively constant 2, 4
Elevated hematocrit (hemoconcentration) directly confirms volume depletion, as intravascular volume loss concentrates red blood cells 1, 3
For a 50-year-old woman, the expected baseline creatinine is approximately 0.9 mg/dL (80 μmol/L), so even "mild" elevation represents significant functional impairment in the setting of volume depletion 5
Immediate Clinical Assessment
Examine for specific volume depletion signs:
Hypotension, poor skin turgor, dry mucous membranes, and low jugular venous pressure indicate hypovolemia requiring fluid administration 1, 6
Urine specific gravity ≥1.030 indicates concentrated urine consistent with appropriate renal response to volume depletion 1
Fractional sodium excretion <1% supports prerenal azotemia, though this finding is present in only a minority of cases and should not be relied upon exclusively 2
Treatment Algorithm
Initiate immediate fluid resuscitation based on severity:
For mild-moderate dehydration with ability to tolerate oral intake: Oral rehydration solution with electrolytes 1
For severe dehydration or inability to tolerate oral intake: IV crystalloid fluids (normal saline or lactated Ringer's), avoiding dextrose-only solutions initially 1
Target fluid replacement to restore intravascular volume while monitoring for resolution of azotemia 1
Monitoring Parameters During Resuscitation
Daily assessment must include:
Body weight (most reliable short-term indicator of fluid status changes) 1, 6
Repeat BUN and creatinine within 24-48 hours to confirm improvement with hydration 1
Urine output monitoring and expect urine specific gravity to decrease as hydration improves 1
Electrolytes, particularly potassium, as underlying causes of volume loss (e.g., diarrhea, vomiting) can cause hypokalemia 1
Expected Response and Red Flags
With appropriate fluid resuscitation:
Creatinine should normalize or near-normalize if this is purely prerenal, typically within 3-4 days 1, 3
BUN should decline in parallel with volume repletion 3
Hematocrit should decrease as intravascular volume is restored 3
Consider nephrology referral if:
Creatinine rises or fails to improve despite adequate hydration (suggests intrinsic renal injury) 1
Development of oliguria, hyperkalemia, or metabolic acidosis 1
Critical Pitfalls to Avoid
Do not assume normal creatinine means normal renal function in elderly or low muscle mass patients, who may have reduced GFR despite "normal" creatinine 1
Avoid nephrotoxic medications (NSAIDs, aminoglycosides, IV contrast) until renal function normalizes 5, 1
Do not over-interpret fractional sodium excretion, as it is <1% in only a minority of prerenal azotemia cases despite classic teaching 2
Recognize that severe disproportionate BUN:Cr elevation is frequently multifactorial and may involve increased protein catabolism, high protein intake, or gastrointestinal bleeding in addition to volume depletion 2