In a one‑year‑old child with a BUN‑to‑creatinine ratio of 73, what does this indicate and how should it be managed?

BUN:Creatinine Ratio of 73 in a One-Year-Old

A BUN:creatinine ratio of 73 in a one-year-old is markedly elevated and most likely indicates severe prerenal azotemia (dehydration/hypovolemia), though intrinsic renal disease or increased protein catabolism must also be considered. 1, 2

Understanding the Elevated Ratio

Normal Pediatric Values

• The normal BUN:creatinine ratio in children is typically 10-34, with younger children having higher ratios than adults due to higher protein intake relative to muscle mass. 3
• A ratio of 73 is approximately 2-7 times the upper limit of normal for this age group, indicating significant pathology. 3

Physiological Context

• BUN is reabsorbed in the proximal tubule (40-50% of filtered load) along with sodium and water, making it highly sensitive to volume status and renal perfusion. 4
• Creatinine is freely filtered but not reabsorbed, making it a more stable marker of glomerular filtration rate. 4
• In states of decreased renal perfusion, enhanced tubular reabsorption of urea occurs while creatinine clearance may remain relatively preserved, leading to disproportionate BUN elevation. 1

Differential Diagnosis (in order of likelihood)

1. Prerenal Azotemia (Most Common)

Look for:

• Diarrhea (especially if profuse/watery), vomiting, decreased oral intake, or fever causing fluid losses 5, 2
• Clinical signs of dehydration: sunken fontanelle, dry mucous membranes, decreased skin turgor, reduced urine output, tachycardia 5
• History of recent illness, particularly gastroenteritis 2

Key point: In severe diarrheal illness (like cholera), the BUN:creatinine ratio may paradoxically be lower than expected for prerenal failure (<15:1) due to massive protein losses, but ratios >40 are more typical. 2

2. Intrinsic Renal Disease

Consider if:

• Oliguria is present at presentation (strongly associated with complicated course and poor outcome) 5
• BUN:creatinine ratio ≤40 in the context of hemolytic uremic syndrome or other thrombotic microangiopathy 5
• Hypertension is present (associated with renal parenchymal disease). 5
• History of recent bloody diarrhea (suggesting diarrhea-associated HUS). 5

3. Increased Protein Catabolism

Evaluate for:

• Sepsis or severe infection (common in critically ill infants). 6
• High protein intake (though less likely to cause ratio of 73 alone). 6
• Gastrointestinal bleeding. 6
• Corticosteroid administration. 6

4. Metabolic Disorders (Rare but Important)

Screen for:

• Citrin deficiency: paradoxically LOW BUN or BUN:creatinine ratio is more typical, making this diagnosis less likely with ratio of 73 7
• Urea cycle disorders: typically present with hyperammonemia rather than isolated BUN elevation. 4

Immediate Management Algorithm

Step 1: Assess Volume Status and Vital Signs

• Measure blood pressure at presentation (hypertension suggests intrinsic renal disease rather than simple dehydration). 5
• Document urine output (oliguria is a red flag for complicated course). 5
• Assess for signs of shock or hemodynamic instability. 6

Step 2: Laboratory Evaluation

Obtain immediately:

• Serum creatinine to calculate actual ratio and estimate GFR using revised Bedside Schwartz equation: GFR (mL/min per 1.73 m²) = 0.413 × [height (cm) / creatinine (mg/dL)] 4, 8, 9
• Complete blood count with peripheral smear (to evaluate for hemolytic anemia/thrombocytopenia suggesting HUS). 5
• Serum electrolytes, particularly potassium. 1
• Urinalysis with microscopy (to distinguish prerenal from intrinsic renal disease) 1
• Lactate dehydrogenase (elevated in HUS). 5

Calculate fractional sodium excretion if possible:

• FENa <1% supports prerenal azotemia (though only 4/11 patients with severe azotemia had this finding in one study). 6

Step 3: Fluid Resuscitation (if prerenal azotemia suspected)

Administer isotonic crystalloid (normal saline or lactated Ringer's) if hypovolemia is present. 1

• Monitor response with serial BUN, creatinine, and electrolytes. 1
• In prerenal azotemia, expect BUN to decrease more rapidly than creatinine with adequate hydration. 1

Step 4: Risk Stratification

High-risk features requiring intensive monitoring/nephrology consultation:

• BUN:creatinine ratio ≤40 (paradoxically suggests intrinsic renal disease like HUS rather than simple dehydration) 5
• Oliguria at presentation. 5
• Hypertension. 5
• Elevated serum creatinine (>0.3 mg/dL increase from baseline if known). 4
• Evidence of hemolysis or thrombocytopenia. 5

Step 5: Ongoing Monitoring

• Recheck BUN and creatinine 1-2 weeks after initial treatment to ensure normalization. 1
• If BUN remains elevated or creatinine worsens despite adequate hydration, refer to pediatric nephrology immediately. 4

Critical Pitfalls to Avoid

1. Do not assume all elevated BUN:creatinine ratios are benign prerenal azotemia—a ratio ≤40 in the setting of acute illness may indicate severe intrinsic renal disease requiring urgent intervention. 5

2. Do not rely on serum creatinine alone to assess renal function in a one-year-old, as it is affected by muscle mass and may not reflect true GFR. 8, 9

3. Do not use adult normative creatinine values—pediatric reference ranges are essential for accurate interpretation. 4

4. Do not overlook multifactorial causes—elderly patients and critically ill children often have multiple contributing factors (infection, poor nutrition, high protein intake). 6

5. In neonates and young infants, remember that serum creatinine may initially reflect maternal levels and baseline renal function determination is often problematic. 4

When to Refer to Nephrology

Immediate referral indicated if: 4

• Oliguria persists despite adequate fluid resuscitation
• Creatinine continues to rise or fails to normalize within 48-72 hours
• BUN:creatinine ratio ≤40 with clinical features of HUS (bloody diarrhea, hemolytic anemia, thrombocytopenia)
• Hypertension is present
• Uncertainty about etiology of azotemia
• Estimated GFR <60 mL/min/1.73 m² persists after rehydration