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ABG arterial blood gas interpretation 2025 clinical guide

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Here is a complete, textbook-grounded guide to ABG interpretation:

Arterial Blood Gas (ABG) - Complete Interpretation Guide

Normal Reference Values

ParameterNormal Range
pH7.35 - 7.45
PaO280 - 100 mmHg
PaCO235 - 45 mmHg
HCO3-22 - 26 mEq/L
SaO295 - 100%
Base excess-2 to +2

5-Step Systematic Interpretation

Step 1 - Assess the pH

  • < 7.35 = Acidemia
  • > 7.45 = Alkalemia
  • 7.35 - 7.45 = Normal (may still have a compensated disorder)

Step 2 - Identify the primary disorder

Acid-base disorders flowchart
pHPaCO2HCO3-Disorder
< 7.35> 45NormalRespiratory acidosis
< 7.35Normal< 22Metabolic acidosis
> 7.45< 35NormalRespiratory alkalosis
> 7.45Normal> 26Metabolic alkalosis

Step 3 - Assess compensation (expected values)

Use the formulas from Henry's Clinical Diagnosis and Management by Laboratory Methods to check if compensation is appropriate:
Primary DisorderCompensation Formula
Metabolic acidosisExpected PaCO2 = ΔHCo3- × 1.2 ± 2 (Winter's formula)
Metabolic alkalosisExpected ΔPCO2 = ΔHCO3- × 0.7 ± 5
Acute respiratory acidosisExpected ΔHCO3- = ΔPaCO2 × 0.07 ± 1.5
Chronic respiratory acidosisExpected ΔHCO3- = ΔPaCO2 × 0.4 ± 3
Acute respiratory alkalosisExpected ΔHCO3- = ΔPaCO2 × 0.2 ± 2.5
Chronic respiratory alkalosisExpected ΔHCO3- = ΔPaCO2 × 0.5 ± 2.5
  • If compensation is less than expected → mixed disorder (second acidosis present)
  • If compensation is greater than expected → second alkalosis present

Step 4 - Calculate the Anion Gap (if metabolic acidosis)

AG = Na+ - (Cl- + HCO3-)
Normal AG: < 12 mEq/L
Correct for hypoalbuminemia: Add 2.5 mEq/L to calculated AG for every 1 g/dL drop in albumin below normal.
AG diagram showing non-AG vs AG metabolic acidosis
High AG metabolic acidosis (MUDPILES):
  • Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethanol, Salicylates
Normal AG (hyperchloremic) metabolic acidosis (HARDUPS):
  • Hyperalimentation, Addison's disease, Renal tubular acidosis, Diarrhea, Ureteral diversion, Pancreatic fistula, Saline infusion

Step 5 - Assess oxygenation

  • PaO2 < 80 mmHg = Hypoxemia
  • Calculate A-a gradient = PAO2 - PaO2
    • PAO2 = (FiO2 × 713) - (PaCO2 / 0.8)
    • Normal A-a gradient: ~4 mmHg (age-adjusted: Age/4 + 4)
    • Widened A-a gradient suggests V/Q mismatch, diffusion impairment, or shunt
    • Normal A-a gradient with hypoxia suggests pure hypoventilation

Quick Summary of Disorders

DisorderpHPaCO2HCO3-Common Causes
Respiratory acidosis↓↑↑ (compensated)COPD, hypoventilation, sedation
Respiratory alkalosis↑↓↓ (compensated)Anxiety, PE, pain, hypoxia
Metabolic acidosis↓↓ (compensated)↓DKA, lactic acidosis, renal failure
Metabolic alkalosis↑↑ (compensated)↑Vomiting, diuretics, excess alkali

Key Clinical Pearls

  • Arterial blood is the gold standard - venous pH correlates closely (±0.05 units) but venous PO2 cannot assess oxygenation - Tintinalli's Emergency Medicine
  • Pulse oximetry cannot detect hypercarbia, especially in patients on supplemental O2. A patient can have significant CO2 retention with a normal SpO2 - Murray & Nadel's Respiratory Medicine
  • Compensation is most effective in respiratory alkalosis (pH often normalizes), then respiratory acidosis, then metabolic acidosis. It is least effective in metabolic alkalosis - Henry's Clinical Diagnosis
  • Mixed disorders are common in the ICU. Always check if compensation matches the formula; deviation signals a second disorder.
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