Ammonia-to-BUN Ratio Calculator

The ammonia-to-BUN ratio is a valuable clinical tool that compares blood ammonia levels to blood urea nitrogen (BUN), providing insight into the liver’s ability to convert toxic ammonia into urea. This ratio helps diagnose and monitor conditions like hepatic encephalopathy, liver failure, and urea cycle disorders.

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Formulas

1. Basic Formula

2. Alternative Units

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Unit Conversions

Substance	Conversion	Factor
Ammonia	µg/dL → µmol/L	× 0.587
	µmol/L → µg/dL	× 1.70
BUN	mg/dL → mmol/L	× 0.357
	mmol/L → mg/dL	× 2.80

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Step-by-Step Calculation

Example 1: Using µg/dL and mg/dL

• Ammonia: 80 µg/dL
• BUN: 12 mg/dL

Convert Ammonia µg/dL → µmol/L = 80 × 0.587 = 46.96 μmol/L

Interpretation: Normal ratio (<10).

Example 2: Using µmol/L and mmol/L

• Ammonia: 50 µmol/L
• BUN: 3.5 mmol/L

Convert BUN mmol/L → mg/dL = 3.5 × 2.80 = 9.8mg/dL

Interpretation: Elevated ratio (≥10), suggesting impaired urea synthesis.

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Key Concepts

1. Ammonia (NH₃)

• Source: Produced from protein metabolism, bacterial action in the gut
• Normal Range:
• Adults: 15–45 µg/dL (9–27 µmol/L)
• Neonates: 64–107 µg/dL (38–64 µmol/L)

2. Blood Urea Nitrogen (BUN)

• Source: End product of ammonia metabolism in the liver
• Normal Range: 6–20 mg/dL (2.1–7.1 mmol/L nitrogen)

3. Ammonia-to-BUN Ratio

• Purpose: Assesses the liver’s urea cycle efficiency
• Normal Ratio: <10 (unitless)
• High Ratio: Suggests impaired urea synthesis

Clinical Interpretation

Normal Ratio (<10)

• Efficient ammonia detoxification
• Suggests non-hepatic causes of hyperammonemia (e.g., transient elevation)

High Ratio (≥10)

1. Liver Dysfunction
   • Cirrhosis
   • Acute liver failure
2. Urea Cycle Disorders
   • Ornithine transcarbamylase (OTC) deficiency
   • Citrullinemia
3. Portosystemic Shunting
   • Ammonia bypasses the liver

Low Ratio (<5)

• High BUN relative to ammonia
• Causes: dehydration, high-protein diet, GI bleeding

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Clinical Applications

1. Hepatic Encephalopathy

• Elevated ratio supports diagnosis
• Guides lactulose/rifaximin therapy

2. Urea Cycle Disorders

• Neonates with ratios >20 require urgent treatment

3. Monitoring Liver Disease

• Rising ratio indicates worsening liver function

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Limitations

• Ammonia: Labile (affected by hemolysis, delayed processing)
• BUN: Influenced by hydration, renal function
• Age Differences: Neonates have higher baseline ammonia

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Key Takeaways

1. Normal Ratio: <10
2. High Ratio (≥10): Suggests liver disease or urea cycle disorder
3. Always correlate with clinical context (e.g., mental status, liver enzymes)

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Possible References Used

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