Acid-Base Calculator

Acid-Base Calculator

Arterial Blood Gas (ABG) Interpreter & Anion Gap Calculator

ABG Input Data
Blood Gas Values
Ref: 7.35 - 7.45
mmHg
Ref: 35 - 45 mmHg
mEq/L
Ref: 22 - 26 mEq/L
Electrolytes (Optional for Anion Gap)
mEq/L
mEq/L

Select clinical context to calculate expected compensation for respiratory disorders.

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Anion Gap

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Delta Ratio

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Clinical Guide to Acid-Base Interpretation

A structured approach to Arterial Blood Gas (ABG) analysis allows clinicians to identify primary physiological disturbances and compensatory mechanisms efficiently.

1. Understanding the Components

Arterial blood gas analysis measures the pH, partial pressure of carbon dioxide (PaCO2), and bicarbonate (HCO3-) to assess the metabolic and respiratory status of a patient.

  • pH: Indicates the overall acidity or alkalinity of the blood. The normal range is 7.35 – 7.45. Values <7.35 indicate acidemia, while >7.45 indicate alkalemia.
  • PaCO2: The respiratory component. It is an acid regulated by ventilation. Normal range is 35 – 45 mmHg.
  • HCO3-: The metabolic component. It is a base regulated by the kidneys. Normal range is 22 – 26 mEq/L.

2. The Step-by-Step Approach

Clinical guidelines, such as those discussed in standard physiology texts and emergency medicine literature, recommend a sequential method to minimize error.

Step 1: Determine pH Status

Is the patient acidemic (pH < 7.35) or alkalemic (pH > 7.45)?

Step 2: Identify the Primary Disorder

Look at the PaCO2 and HCO3. Which one matches the direction of the pH change?

  • Respiratory Acidosis: Low pH, High PaCO2.
  • Respiratory Alkalosis: High pH, Low PaCO2.
  • Metabolic Acidosis: Low pH, Low HCO3.
  • Metabolic Alkalosis: High pH, High HCO3.
Step 3: Calculate Compensation

Is the body compensating appropriately? If the measured value falls outside the expected calculated range, a mixed disorder exists.

Note: The body never fully over-compensates to normalize pH in primary disorders.

3. Formulas Used in This Calculator

Metabolic Acidosis (Winter's Formula)

To determine if respiratory compensation is adequate:

Expected PaCO2 = (1.5 × HCO3) + 8 ± 2

If measured PaCO2 is higher than expected, there is a concomitant Respiratory Acidosis. If lower, there is a concomitant Respiratory Alkalosis.

Metabolic Alkalosis

Expected compensation is calculated as:

Expected PaCO2 = 0.7 × (HCO3 - 24) + 40 ± 2

Respiratory Disorders

Compensation depends on acuity (Acute vs. Chronic) because renal compensation takes days to fully manifest.

Disorder Acute Change Chronic Change
Resp. Acidosis HCO3 ↑1 per 10 mmHg ↑CO2 HCO3 ↑4 per 10 mmHg ↑CO2
Resp. Alkalosis HCO3 ↓2 per 10 mmHg ↓CO2 HCO3 ↓5 per 10 mmHg ↓CO2

4. The Anion Gap & Delta Ratio

If a Metabolic Acidosis is present, the Anion Gap (AG) helps differentiate the etiology.

Formula: AG = Na+ - (Cl- + HCO3-)
Normal Range: 8 – 12 mEq/L

An elevated AG (>12) indicates the presence of unmeasured anions (High Anion Gap Metabolic Acidosis - HAGMA). Common causes can be remembered via the mnemonic MUDPILES (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acid, Ethylene glycol, Salicylates).

The Delta Ratio

The Delta Ratio compares the increase in Anion Gap to the decrease in Bicarbonate. It helps identify mixed metabolic disorders in the setting of HAGMA.

Delta Ratio = (AG - 12) / (24 - HCO3)
  • < 0.4: Hyperchloremic (Normal AG) acidosis.
  • 0.4 – 0.8: Combined HAGMA and Normal AG acidosis.
  • 1 – 2: Pure HAGMA (Typical for DKA).
  • > 2: HAGMA with concurrent Metabolic Alkalosis (or pre-existing high bicarbonate).

Disclaimer: This tool is for educational and reference purposes only. It should not replace clinical judgment or official medical advice. Always interpret lab results in the context of the patient's full clinical presentation.