Blood Gas Analysis (BGA)

More commonly called Arterial Blood Gas (ABG) analysis—is a laboratory test that measures how well the lungs are oxygenating blood, how effectively carbon dioxide is being removed, and how the body is regulating acid–base balance.

What does it measure?

1. pH

  • Indicates acidity or alkalinity of blood

  • Normal: 7.35–7.45

  • Low pH → acidosis

  • High pH → alkalosis

2. PaO₂ (partial pressure of oxygen)

  • Reflects oxygenation of arterial blood

  • Normal: 80–100 mmHg

  • Low PaO₂ → hypoxemia

3. PaCO₂ (partial pressure of carbon dioxide)

  • Reflects adequacy of ventilation

  • Normal: 35–45 mmHg

  • High PaCO₂ → respiratory acidosis

  • Low PaCO₂ → respiratory alkalosis

4. HCO₃⁻ (bicarbonate)

  • Main metabolic buffer

  • Normal: 22–26 mEq/L

  • Abnormal values suggest metabolic acid–base disorders

5. O₂ Saturation (SaO₂)

  • Percentage of hemoglobin bound to oxygen

  • Normal: ≥95%

Where is the blood taken from?

  • Usually arterial blood, most commonly from:

    • Radial artery

    • Femoral artery

    • Brachial artery

(Venous blood gas can be used in limited settings but is not a full substitute.)

What is it used for?

Blood gas analysis is essential in assessing:

  • Respiratory failure (e.g., asthma, COPD, ARDS)

  • Metabolic disorders (e.g., diabetic ketoacidosis, renal failure)

  • Sepsis and shock

  • Critical care monitoring

  • Ventilator management

  • Unexplained dyspnea or altered mental status

How results are interpreted (simplified)

  1. Look at pH → acidemia or alkalemia

  2. Check PaCO₂ → respiratory cause?

  3. Check HCO₃⁻ → metabolic cause?

  4. Determine if compensation is present

  5. Assess oxygenation (PaO₂, SaO₂)

Example

  • pH: 7.28

  • PaCO₂: 55 mmHg

  • HCO₃⁻: 26 mEq/L

➡️ Primary respiratory acidosis (e.g., hypoventilation)

In one sentence:

Blood gas analysis evaluates oxygenation, ventilation, and acid–base balance, providing rapid, life-saving information in acute and critical illness.

If you want, I can also:

  • Walk you through step-by-step ABG interpretation

  • Provide board-style practice questions

  • Compare ABG vs VBG

  • Create a clinical algorithm or flowchart

 

Clinical Laboratory Tests

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