Ammonia-to-Urea Ratio Calculator

The Ammonia-to-Urea Ratio Calculator is a clinical tool used to evaluate the liver’s ability to convert toxic ammonia into urea. This ratio helps diagnose conditions like hepatic encephalopathy, urea cycle disorders, and liver failure by assessing the balance between nitrogen waste production and elimination.

---

Definition

Ammonia-to-Urea Ratio quantifies the relationship between blood ammonia (NH₃) and urea (BUN) levels. A high ratio suggests impaired urea synthesis, often due to liver dysfunction or metabolic disorders.

---

Core Formula

The basic formula for the ammonia-to-urea ratio is:

Formula Variations by Units

1. Using µmol/L (Ammonia) and mmol/L (Urea):

2. Using µg/dL (Ammonia) and mg/dL (Urea):

---

Unit Conversions

Substance	Conversion	Factor
Ammonia	µg/dL → µmol/L	× 0.587
	µmol/L → µg/dL	× 1.70
Urea	mg/dL → mmol/L	× 0.1665
	mmol/L → mg/dL	× 6.0

---

Step-by-Step Calculation Examples

Example 1: Using µmol/L and mmol/L

• Ammonia: 60 µmol/L
• Urea: 5 mmol/L

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

Example 2: Using µg/dL and mg/dL

• Ammonia: 85 µg/dL
• Urea: 15 mg/dL

• Ammonia (µmol/L)=85×0.587=49.895 μmol/L
• Urea (mmol/L)=15×0.1665=2.4975 mmol/L

Interpretation: Normal ratio (typically <10).

---

Normal Values

• Ammonia:
  • Adults: 15–45 µg/dL (9–27 µmol/L)
  • Neonates: 64–107 µg/dL (38–64 µmol/L)
• Urea (BUN):
  • Adults: 7–20 mg/dL (2.5–7.1 mmol/L)
• Ammonia-to-Urea Ratio:
  • Normal: <10 (unitless)
  • Elevated: ≥10 (indicates pathology)

---

Interpretation

• High Ratio (≥10):
• Liver dysfunction (cirrhosis, acute liver failure).
• Urea cycle disorders (genetic defects in ammonia metabolism).
• Portosystemic shunting (ammonia bypasses liver detoxification).
• Low Ratio (<10):
• Normal or increased urea production (high-protein diet, dehydration).

---

Clinical Relevance

1. Hepatic Encephalopathy:
   Elevated ammonia with a high ratio confirms impaired detoxification.
2. Urea Cycle Disorders:
   Neonates with hyperammonemia and low urea show ratios >20.
3. Monitoring Therapy:
   Tracks response to lactulose, rifaximin, or nitrogen scavengers.

---

Limitations

• Ammonia levels are labile: Affected by hemolysis, delayed processing, or exercise.
• Urea influenced by non-hepatic factors: Hydration, renal function, and diet.
• Age-dependent norms: Neonates naturally have higher ammonia levels.

---

Key Takeaways

1. The ratio compares ammonia (toxic) to urea (non-toxic nitrogen waste).
2. Normal ratio: <10; elevated values suggest liver or metabolic disease.
3. Always pair with clinical context (e.g., mental status, liver enzymes).

---

Possible References Used

---