What are the normal measurement ranges for the components of an Arterial Blood Gas (ABG)?

Normal Measurement Ranges for Arterial Blood Gas Components

The normal ranges for arterial blood gas (ABG) components are: pH 7.35-7.45, PaCO2 35-45 mmHg (4.7-6.0 kPa), PaO2 80-100 mmHg (10.6-13.3 kPa), HCO3- 22-26 mEq/L, base excess -2 to +2 mEq/L, and oxygen saturation 95-100%. 1

Complete ABG Reference Ranges

Component	Normal Range
pH	7.35-7.45
PaCO2	35-45 mmHg (4.7-6.0 kPa)
PaO2	80-100 mmHg (10.6-13.3 kPa)
HCO3-	22-26 mEq/L
Base Excess	-2 to +2 mEq/L
Oxygen Saturation	95-100%
Alveolar-arterial gradient	4-8 mmHg (increases with age)

Additional ABG Components and Values

For patients with specific conditions, other ABG parameters may be relevant:

• Carboxyhemoglobin (COHb):

  • Non-smokers: 3-4% or less 2
  • Smokers: typically 3-5%, up to 10% in heavy smokers 2

• Lactate: Normal range is typically 0.5-2.0 mmol/L

• Electrolytes that may be measured on blood gas analyzers:

  • Sodium (Na+): 135-145 mmol/L
  • Potassium (K+): 3.5-5.0 mmol/L
  • Ionized calcium (iCa): 1.1-1.3 mmol/L

Clinical Interpretation Framework

When interpreting ABGs, follow this systematic approach:

1. Evaluate pH: Determine if acidemia (pH < 7.35) or alkalemia (pH > 7.45) is present
2. Identify primary disorder:
   • Respiratory acidosis: ↓ pH, ↑ PaCO2
   • Respiratory alkalosis: ↑ pH, ↓ PaCO2
   • Metabolic acidosis: ↓ pH, ↓ HCO3-
   • Metabolic alkalosis: ↑ pH, ↑ HCO3-
3. Assess compensation: Determine if appropriate compensatory mechanisms are present
4. Evaluate oxygenation: Assess PaO2 and oxygen saturation

Important Clinical Considerations

• When monitoring patients with COPD, a PaO2 of at least 7.5 kPa (56 mmHg) without a fall in pH below 7.26 is the aim of oxygen therapy 2

• For patients with suspected CO poisoning, standard pulse oximeters cannot differentiate carboxyhemoglobin, potentially giving falsely normal readings 2

• In patients with hypercapnic respiratory failure, arterial blood pH < 7.35 with PaCO2 > 6.0 kPa (45 mmHg) indicates the need for consideration of ventilatory support 2

• For patients with acute ischemic priapism, corporal blood gas values typically show PO2 < 30 mmHg, PCO2 > 60 mmHg, and pH < 7.25, which differ significantly from normal arterial values 2

• ABG sampling should be performed before and after 1 hour on the intended therapeutic flow rate of oxygen for patients with COPD to ensure oxygen therapy isn't causing respiratory decompensation 2

Common Pitfalls in ABG Interpretation

• Relying solely on pulse oximetry without ABG confirmation in critical situations
• Failing to consider the impact of supplemental oxygen on readings
• Not accounting for temperature effects on blood gas values
• Overlooking the presence of carboxyhemoglobin, which can falsely elevate SpO2 readings 1
• Misinterpreting mixed acid-base disorders as simple disorders with compensation

By understanding these normal ranges and interpretation principles, clinicians can effectively use ABG analysis to guide diagnosis and treatment of respiratory, metabolic, and mixed disorders.