Blood Urea Nitrogen in Paediatrics

Also written as Urea.

BUN reflects renal handling of nitrogenous waste and is influenced by hydration, protein intake and renal function. In children it helps differentiate prerenal, intrinsic and post‑renal causes of acute kidney injury.

Definition and diagnosis

BUN is the concentration of urea nitrogen in serum, produced by hepatic conversion of ammonia. Normal range 1.5–5.5 mmol/L (5–20 mg/dL).

• BUN/Creatinine ratio: Is a prerenal marker; >20 suggests hypovolaemia or dehydration.

• Measured by enzymatic assay; often reported with creatinine.

Severity and staging

BUN alone does not stage AKI; use KDIGO creatinine criteria. However a very high BUN (>25 mmol/L) with normal creatinine may indicate severe prerenal azotemia.

• BUN 10–24.9 mmol/L = mild‑moderate
• BUN ≥25 mmol/L = severe

Causes, most likely first

In low‑resource settings the commonest causes are dehydration, diarrhoea, malaria, HIV‑associated nephropathy, and herbal nephrotoxins.

• Dehydration/volume depletion
• Severe diarrhoea
• Malaria (especially cerebral)
• HIV‑associated nephropathy
• Herbal nephrotoxins (e.g., Aristolochia)

How it presents

Elevated BUN may be asymptomatic but can accompany oliguria, lethargy, vomiting, or altered consciousness if severe.

• Oliguria or anuria
• Lethargy or confusion
• Vomiting

Investigations

BUN is part of a basic metabolic panel. Additional tests guide cause and severity.

Red flags and complications

Severe azotemia can precipitate cerebral oedema, hyperkalaemia, metabolic acidosis and death.

• BUN >25 mmol/L with oliguria
• Hyperkalaemia >5.5 mmol/L
• Metabolic acidosis (pH <7.2)

Management

Treat the underlying cause, ensure adequate hydration, monitor renal function and electrolytes, and avoid nephrotoxic drugs.

• Rehydrate with isotonic saline (if prerenal)
• Treat malaria or diarrhoea
• Stop herbal nephrotoxins
• Avoid NSAIDs and aminoglycosides
• Correct hyperkalaemia with insulin/glucose or calcium gluconate

Follow-up and prevention

Repeat BUN/creatinine after 24–48 hours to assess response. Educate on hydration, safe herbal use and early treatment of diarrhoea.

• Repeat labs
• Patient education

Often confused with

Serum creatinine elevation may mimic BUN rise but indicates reduced GFR; BUN can rise with high protein intake.

• High protein diet
• Dehydration

Where results can mislead

BUN can be falsely low in malnutrition or high in liver disease; creatinine may be low in children with low muscle mass.

• Low muscle mass in infants
• Liver failure

Questions an examiner may ask

• A 6‑year‑old presents with vomiting and oliguria after a diarrhoeal episode. BUN is 28 mmol/L, creatinine 1.2 mg/dL. What is the most likely diagnosis? — Prerenal azotemia due to volume depletion; treat with isotonic saline and monitor urine output.
• A child with HIV has rising BUN and creatinine. Which investigation would best confirm the cause? — Urine protein/creatinine ratio and renal ultrasound to look for focal segmental glomerulosclerosis.
• A 4‑year‑old with malaria has BUN 30 mmol/L and normal electrolytes. What is the next step? — Rehydrate aggressively with IV fluids and treat malaria; monitor for hyperkalaemia.
• A child on herbal medicine presents with rising BUN. What should you do? — Discontinue the herbal preparation, assess renal function, and consider nephrotoxicity.
• A 2‑year‑old with dehydration has BUN 18 mmol/L and creatinine 0.4 mg/dL. Why is BUN still high? — BUN rises with volume depletion; creatinine remains low due to low muscle mass.
• A child with suspected post‑renal obstruction has normal BUN but high creatinine. What explains this? — Post‑renal obstruction causes a sudden rise in creatinine before BUN rises; early ultrasound is indicated.

When to seek help

• Severe oliguria or anuria
• Hyperkalaemia >5.5 mmol/L
• Metabolic acidosis (pH <7.2)
• Altered consciousness

Quick questions

Why is BUN/Cr ratio >20 suggestive of dehydration?

Because urea reabsorption increases with volume depletion, raising BUN more than creatinine.

Can a high BUN be caused by a high protein diet?

Yes, but the rise is usually modest and not accompanied by oliguria or electrolyte disturbance.

When should I stop a child on NSAIDs?

If BUN is rising or creatinine is >1.5 mg/dL, stop NSAIDs to avoid further renal injury.