Elevated BUN with Normal Creatinine in a 12-Year-Old
The most likely etiology is dehydration or volume depletion causing prerenal azotemia, and initial management should focus on assessing hydration status and providing fluid resuscitation if indicated. 1, 2
Most Likely Etiology
Prerenal azotemia from dehydration is the primary consideration when BUN is elevated but creatinine remains normal in a pediatric patient. 1, 2 This occurs because:
- Reduced intravascular volume decreases renal perfusion, triggering enhanced urea reabsorption in the proximal tubule (40-50% of filtered urea) while creatinine remains relatively stable 1
- The disproportionate rise in BUN compared to creatinine creates an elevated BUN-to-creatinine ratio, which is the hallmark of prerenal states 1, 2
- Volume depletion concentrates the blood, raising measured BUN levels 1
Other Potential Causes to Consider
While dehydration is most common, also evaluate for:
- Increased protein catabolism from fever, infection, or stress 2, 3
- High protein intake or recent dietary changes 3
- Gastrointestinal bleeding (occult or overt) 2, 3
- Cardiac dysfunction reducing renal perfusion 2, 4
Initial Assessment
Clinical Evaluation for Dehydration
Examine for specific signs of volume depletion: 2, 4
- Dry mucous membranes
- Reduced skin turgor
- Orthostatic hypotension (if age-appropriate to measure)
- Decreased urine output
- Weight loss (if recent weights available)
Laboratory Workup
Obtain the following tests to characterize the azotemia: 5
- Complete metabolic panel including electrolytes, bicarbonate, and glucose 5
- Urinalysis to assess for proteinuria (>2+ by dipstick), red blood cell casts, or deformed red blood cells that would suggest glomerular disease rather than prerenal azotemia 5
- Urine culture if infection suspected 5
- Complete blood count with platelets 5
- Spot urine calcium-to-creatinine ratio if hypercalciuria is a consideration 5
Calculate BUN-to-Creatinine Ratio
A ratio >20:1 strongly suggests prerenal azotemia rather than intrinsic kidney disease. 1, 3 In pure dehydration, creatinine elevation is typically mild and resolves with rehydration. 1
Initial Management
Fluid Resuscitation
If clinical signs of dehydration are present, administer isotonic saline at 10-20 mL/kg/h for pediatric patients. 2, 4 This improves renal perfusion and should normalize BUN if the cause is purely prerenal. 4
Monitoring Response
- Recheck BUN and creatinine after adequate rehydration to confirm resolution 1, 2
- Monitor urine output during rehydration as an indicator of improving renal perfusion 1
- Follow serial BUN measurements until normalization 2, 4
- Assess short-term changes in fluid status by measuring changes in body weight 4
Distinguishing Prerenal from Intrinsic Kidney Disease
Features Favoring Prerenal Azotemia
- Disproportionate BUN elevation with normal or minimally elevated creatinine 1, 2
- Absence of proteinuria, hematuria, or abnormal urinary sediment 1
- Resolution of BUN elevation after rehydration 1
Features Suggesting Intrinsic Kidney Disease
- Proteinuria (>2+ by dipstick), red blood cell casts, or deformed red blood cells on urinalysis suggest glomerulonephritis and require different evaluation 5
- Proportional increases in both BUN and creatinine 1
- Persistent elevation after adequate rehydration requires further investigation for underlying kidney disease 1
Critical Pitfalls to Avoid
- Do not assume elevated BUN always indicates kidney dysfunction when creatinine is normal – prerenal causes are common and reversible 4
- Do not overlook infection as a contributing factor – 14 of 19 patients with severe disproportionate BUN elevation in one study had concurrent infection 3
- Do not delay rehydration while awaiting laboratory results if clinical dehydration is evident 2
- Do not miss occult gastrointestinal bleeding as a cause of elevated BUN from protein load 2, 3
When to Pursue Further Evaluation
If BUN remains elevated despite adequate rehydration, or if urinalysis shows proteinuria, hematuria, or casts, consider: 5