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Serum Urea and Creatinine

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Serum Urea and Creatinine

Serum urea and creatinine are key markers of kidney function and are often measured together to assess renal health, hydration status, and metabolic function.

1. Urea (Blood Urea Nitrogen – BUN)

Normal Range:

  • Urea (Serum): 10–40 mg/dL (varies by lab and diet)
  • Blood Urea Nitrogen (BUN): 6–20 mg/dL (BUN is a form of urea measurement)

Causes of Increased Urea (Uremia or Azotemia):

  1. Pre-Renal Causes (Reduced Blood Flow to Kidneys)

    • Dehydration, shock, heart failure, severe blood loss
    • High-protein diet, increased protein breakdown (burns, sepsis, steroids)

  2. Renal Causes (Intrinsic Kidney Disease)

    • Acute kidney injury (AKI), chronic kidney disease (CKD)
    • Glomerulonephritis, tubulointerstitial nephritis

  3. Post-Renal Causes (Urinary Obstruction)

    • Kidney stones, prostate enlargement, tumors

Causes of Decreased Urea:

  • Low protein diet, liver disease (since the liver synthesizes urea), overhydration, pregnancy

2. Serum Creatinine

Normal Range:

  • Men: 0.7–1.3 mg/dL
  • Women: 0.6–1.1 mg/dL
  • Children: 0.3–0.7 mg/dL

Causes of Increased Creatinine:

  1. Acute or Chronic Kidney Disease
  2. Dehydration or Shock (Reduced Kidney Perfusion)
  3. Muscle Breakdown (Rhabdomyolysis, Trauma, Muscular Dystrophy)
  4. High Protein Intake or Supplement Use (Creatine supplements)
  5. Certain Medications (NSAIDs, ACE inhibitors, aminoglycosides, contrast dyes)

Causes of Decreased Creatinine:

  • Muscle wasting (elderly, malnutrition)
  • Liver disease
  • Pregnancy (due to increased renal clearance)

3. Urea/Creatinine Ratio

Normal Ratio:

  • 10:1 to 20:1 (in mg/dL)

Interpretation of Urea/Creatinine Ratio:

  • High Ratio (>20:1): Suggests pre-renal causes (dehydration, heart failure, GI bleeding, high-protein diet).
  • Normal Ratio (10:1–20:1): Seen in intrinsic kidney disease.
  • Low Ratio (<10:1): Liver disease, low protein intake, malnutrition, SIADH (Syndrome of Inappropriate ADH).

Clinical Importance:

  • Kidney Function Monitoring: Elevated creatinine and urea suggest renal dysfunction.
  • Dehydration vs. Renal Disease: A high urea/creatinine ratio helps differentiate dehydration from intrinsic kidney disease.
  • Liver Disease Assessment: Low urea levels may indicate liver dysfunction.