ABG Calculator

Category: Other Health
- September 18, 2025
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Analyse and interpret arterial blood gas (ABG) results to assess acid-base disturbances, oxygenation status, and ventilation adequacy. This calculator helps healthcare professionals diagnose respiratory and metabolic disorders based on ABG values.

Enter ABG Values

Normal range: 7.35-7.45
Normal range: 35-45 mmHg
Normal range: 22-26 mEq/L
Normal range: 80-100 mmHg
Normal range: 135-145 mEq/L
Normal range: 96-106 mEq/L
Normal range: 3.5-5.0 g/dL
%
Normal range: 95-100%
%
Room air: 21%, Supplemental O₂: 24-100%
Normal range: 0-4000 meters

Understanding ABG Interpretation

Arterial blood gas analysis is an essential tool for assessing a patient's acid-base status, oxygenation, and ventilation. Interpretation involves evaluating pH, PaCO₂, HCO₃⁻, and other parameters to identify respiratory and metabolic disorders.

Normal ABG Values
  • pH: 7.35-7.45
  • PaCO₂: 35-45 mmHg
  • HCO₃⁻: 22-26 mEq/L
  • PaO₂: 80-100 mmHg (decreases with age)
  • SaO₂: 95-100%
  • Base Excess: -2 to +2 mEq/L
Primary Acid-Base Disorders
Disorder pH PaCO₂ HCO₃⁻
Respiratory Acidosis ↓ ↑ Normal (acute)
↑ (compensated)
Respiratory Alkalosis ↑ ↓ Normal (acute)
↓ (compensated)
Metabolic Acidosis ↓ Normal (acute)
↓ (compensated)		 ↓
Metabolic Alkalosis ↑ Normal (acute)
↑ (compensated)		 ↑
Compensatory Mechanisms
  • Respiratory Compensation: Changes in respiratory rate and depth adjust PaCO₂ levels to compensate for metabolic disorders.
  • Metabolic Compensation: Renal mechanisms adjust bicarbonate levels to compensate for respiratory disorders.
  • Expected Compensation:
    • For Metabolic Acidosis: PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2
    • For Metabolic Alkalosis: PaCO₂ increases by 0.7 mmHg for each 1 mEq/L increase in HCO₃⁻
    • For Respiratory Acidosis:
      • Acute: HCO₃⁻ increases by 1 mEq/L for each 10 mmHg increase in PaCO₂
      • Chronic: HCO₃⁻ increases by 3.5 mEq/L for each 10 mmHg increase in PaCO₂
    • For Respiratory Alkalosis:
      • Acute: HCO₃⁻ decreases by 2 mEq/L for each 10 mmHg decrease in PaCO₂
      • Chronic: HCO₃⁻ decreases by 5 mEq/L for each 10 mmHg decrease in PaCO₂
Anion Gap

The anion gap helps distinguish between different causes of metabolic acidosis:

  • Anion Gap = Na⁺ - (Cl⁻ + HCO₃⁻)
  • Normal range: 8-12 mEq/L (or 10-14 mEq/L depending on laboratory methods)
  • High anion gap metabolic acidosis (HAGMA): Can be remembered by the mnemonic MUDPILES
    • M - Methanol
    • U - Uremia (renal failure)
    • D - Diabetic ketoacidosis
    • P - Paraldehyde, Phenformin, Propylene glycol
    • I - Isoniazid, Iron, Inborn errors of metabolism
    • L - Lactic acidosis
    • E - Ethylene glycol
    • S - Salicylates, Starvation ketoacidosis
  • Normal anion gap metabolic acidosis (NAGMA): Often due to GI bicarbonate loss, renal tubular acidosis, or administration of chloride-containing solutions
Oxygenation Assessment
  • A-a Gradient: Difference between alveolar (PAO₂) and arterial (PaO₂) oxygen tension
    • A-a Gradient = PAO₂ - PaO₂
    • PAO₂ = (FiO₂ × (Patm - PH₂O)) - (PaCO₂ / RQ)
    • Where: RQ (Respiratory Quotient) ≈ 0.8, Patm = 760 mmHg at sea level, PH₂O = 47 mmHg
    • Normal A-a Gradient: 5-10 mmHg (increases with age: 2.5 + 0.21 × age)
  • PaO₂/FiO₂ (P/F) Ratio: Used to assess severity of hypoxemia
    • Normal: > 400 mmHg
    • Mild hypoxemia: 300-400 mmHg
    • Moderate hypoxemia: 200-300 mmHg
    • Severe hypoxemia: < 200 mmHg (ARDS criteria when accompanied by bilateral infiltrates)

Disclaimer: This ABG calculator is intended for educational and informational purposes only. It should be used as a supplementary tool by qualified healthcare professionals and not as a replacement for clinical judgment. Always correlate calculator results with the patient's clinical presentation and consult appropriate medical resources and specialists for clinical decisions.

What Is the ABG Calculator?

The Arterial Blood Gas (ABG) Calculator is a clinical tool used to interpret blood test values that reflect how well a patient's lungs and kidneys are managing acid-base balance, oxygenation, and carbon dioxide levels. This calculator assists healthcare professionals in assessing respiratory function, metabolic disorders, and evaluating a patient's oxygenation status.

While this tool is most relevant in emergency medicine, intensive care, and respiratory therapy, it can also help clarify the nature of acid-base disturbances for medical students and trainees.

How to Use the ABG Calculator

Follow these simple steps to use the ABG Calculator effectively:

  • Enter values for pH, PaCO₂, HCO₃⁻, and PaO₂ from the patient’s ABG report.
  • Provide Other optional values such as sodium (Na⁺), chloride (Cl⁻), albumin, SaO₂, FiO₂, and altitude if available.
  • Check applicable clinical conditions like COPD or intubation status for more accurate results.
  • Click the “Analyze ABG” button to view a complete breakdown of the interpretation.

The results include:

  • Primary acid-base diagnosis
  • Compensation status
  • Anion gap and oxygenation assessment
  • Visual data with graphs
  • Clinical implications and diagnostic considerations

Key Formulas Used in ABG Analysis

1. Anion Gap: Na⁺ − (Cl⁻ + HCO₃⁻)

2. Corrected Anion Gap: Anion Gap + 2.5 × (4 − Albumin)

3. Base Excess: (HCO₃⁻ − 24) + 2.3 × (pH − 7.4)

4. Alveolar Oxygen Tension (PAO₂): FiO₂ × (Pb − PH₂O) − (PaCO₂ ÷ 0.8)

5. A-a Gradient: PAO₂ − PaO₂

6. P/F Ratio: PaO₂ ÷ FiO₂

7. Henderson-Hasselbalch Equation: pH = 6.1 + log([HCO₃⁻] ÷ (0.03 × PaCO₂))

Why Is ABG Interpretation Important?

ABG interpretation is essential for:

  • Identifying respiratory vs. metabolic causes of acidosis or alkalosis
  • Determining whether a disorder is acute, chronic, or compensated
  • Assessing oxygenation and risk for respiratory failure or ARDS
  • Guiding urgent interventions such as ventilator adjustments or fluid management

Related Health Calculators

The ABG Calculator shares its clinical utility with other specialised tools used by healthcare providers:

Frequently Asked Questions (FAQ)

What is a normal arterial blood pH?

The normal range is 7.35 to 7.45. Values below this suggest acidosis; values above suggest alkalosis.

What does a high anion gap indicate?

It suggests the presence of unmeasured acids in the blood, common in conditions like lactic acidosis or diabetic ketoacidosis.

Can this calculator replace clinical judgment?

No. It supports decision-making but must be used alongside clinical assessment and other diagnostic tools.

Is this tool useful for patients with chronic conditions?

Yes. It allows adjustments for conditions like COPD and prolonged acid-base disorders to provide more relevant interpretations.

Can I use this for educational purposes?

Absolutely. Medical students and residents often use ABG calculators for practice in acid-base interpretation.

How This Calculator Helps

This ABG Calculator enhances understanding and supports quick analysis of acid-base imbalances, helping clinicians identify issues that require urgent attention. Similar to tools like the GFR Calculator or BAC estimation apps that assess kidney function or blood alcohol levels, this tool brings clarity to another critical aspect of patient physiology: acid-base status and oxygenation.

It also complements tools like the Corrected Calcium Calculator or Body Surface Area Chart by providing relevant metabolic data that influence overall patient care. Whether you're adjusting a ventilator setting or evaluating a patient with unexplained confusion, this calculator is a practical, time-saving aid.