This guide provides a comprehensive 8-step framework for ABG interpretation — from pH assessment through compensatory responses, anion gap analysis, delta-delta ratio, and the A-a gradient. Includes 15 fully worked clinical examples spanning COPD, DKA, salicylate toxicity, PE, sepsis, TCA overdose, and more.
Step 1 — Basics & Normal Values
An Arterial Blood Gas (ABG) provides direct measurement of gas exchange and acid-base status, giving immediate quantitative information about ventilation, oxygenation, and metabolic function.
The 8-Step Framework
Step 2 — pH Assessment
| pH | Interpretation | Clinical Significance |
|---|---|---|
| <7.35 | Acidemia | Respiratory or metabolic acidosis is primary driver |
| 7.35–7.45 | Normal | May still have mixed disorder — don't stop here |
| >7.45 | Alkalemia | Respiratory or metabolic alkalosis is primary driver |
⚠️ A normal pH does not rule out pathology. Compensated or mixed disorders can normalize pH while underlying abnormalities persist. Always examine all values.
Step 3 — Identify the Primary Disorder
| pH | PaCO₂ | HCO₃⁻ | Primary Disorder |
|---|---|---|---|
| Low (<7.35) | High (>45) | Normal/High | Respiratory Acidosis |
| Low (<7.35) | Normal/Low | Low (<22) | Metabolic Acidosis |
| High (>7.45) | Low (<35) | Normal/Low | Respiratory Alkalosis |
| High (>7.45) | Normal/High | High (>26) | Metabolic Alkalosis |
Step 4 — Compensation Formulas
Compensation is the body's attempt to restore pH by the opposite system. It is never complete — it moves pH back toward normal, but not to 7.40.
Actual < expected → additional respiratory alkalosis
Actual > expected → additional respiratory acidosis
Lungs hypoventilate to retain CO₂ and buffer rise in pH
Chronic: +3.5 mEq HCO₃⁻ per 10 mmHg ↑ PaCO₂
Chronic = established renal adaptation (days)
Chronic: −5 mEq HCO₃⁻ per 10 mmHg ↓ PaCO₂
If more than expected → mixed process
Step 5 — Anion Gap & GOLD MARK
GOLD MARK — High AG Metabolic Acidosis Causes
| Letter | Cause | Key Mechanism |
|---|---|---|
| G | Glycols (ethylene/propylene) | Metabolic acidosis + osmolal gap |
| O | Oxoproline (acetaminophen toxicity) | Pyroglutamic acid accumulation |
| L | L-Lactate (shock, sepsis, ischemia) | Anaerobic metabolism |
| D | D-Lactate (short bowel syndrome) | Bacterial fermentation |
| M | Methanol | Formic acid production |
| A | Aspirin (salicylate toxicity) | Uncouples oxidative phosphorylation |
| R | Renal failure (uremia) | Retention of sulfate, phosphate |
| K | Ketones (DKA, AKA, starvation) | Ketoacid accumulation |
Normal AG Metabolic Acidosis (NAGMA) — Causes
Mnemonic: HARDUPS — Hyperalimentation, Acetazolamide, Renal tubular acidosis, Diarrhea, Ureteroenteric fistula, Pancreatic fistula, Saline excess (hyperchloremic)
Step 6 — Delta-Delta Ratio
Used in high anion gap metabolic acidosis to detect a concurrent second acid-base disorder hidden beneath the primary process.
| ΔAG Result | Interpretation |
|---|---|
| −6 to +6 (≈ 0) | Pure high AG metabolic acidosis |
| < −6 | Concurrent NAGMA also present |
| > +6 | Concurrent metabolic alkalosis also present |
DKA + Vomiting
DKA → high AG metabolic acidosis (AG = 22). Vomiting → metabolic alkalosis (HCO₃⁻ higher than expected). ΔAG will be >+6, revealing the alkalosis "hidden" by the acidosis.
Step 7 — A-a Gradient & Oxygenation
Simplified on room air: PAO₂ ≈ 150 − (PaCO₂ / 0.8)
| A-a Gradient | Interpretation | Causes |
|---|---|---|
| Normal (<15–20 mmHg) | Hypoventilation alone | CNS depression, opioids, neuromuscular |
| Elevated (>20 mmHg) | V/Q mismatch, shunt, diffusion defect | PE, ARDS, pneumonia, pulmonary edema |
15 Clinical Examples
Chronic Respiratory Acidosis
ABG: pH 7.32, PaCO₂ 68, HCO₃⁻ 34, PaO₂ 55
Analysis: Primary respiratory acidosis. Chronic: expected HCO₃⁻ = 24 + (3.5 × 2.8) ≈ 34 ✓ → fully compensated chronic.
Management: Controlled O₂ (target SpO₂ 88–92%), NIV/BiPAP, bronchodilators. Avoid high-flow O₂ (hypoxic drive).
High AG Metabolic Acidosis
ABG: pH 7.12, PaCO₂ 20, HCO₃⁻ 6, AG 28
Analysis: Primary metabolic acidosis. Winter's: expected PaCO₂ = (1.5×6)+8 = 17 ± 2 → actual 20 = appropriate compensation.
Management: IV fluids (NS), insulin, K⁺ replacement. HCO₃⁻ only if pH <6.9. Monitor K⁺ closely.
Mixed High AG Metabolic Acidosis + Respiratory Alkalosis
ABG: pH 7.46, PaCO₂ 22, HCO₃⁻ 15, AG 22
Analysis: Mixed disorder! Salicylates → both respiratory alkalosis (stimulates respiratory center) AND metabolic acidosis (uncouples oxidative phosphorylation). Normal pH masks severity.
Management: IV NaHCO₃, urinary alkalinization, hemodialysis if severe.
Respiratory Alkalosis with Elevated A-a Gradient
ABG: pH 7.51, PaCO₂ 28, HCO₃⁻ 22, PaO₂ 62
Analysis: Primary respiratory alkalosis (hyperventilation from pain/hypoxia). A-a gradient elevated → V/Q mismatch from PE. Normal HCO₃⁻ → acute, no renal compensation yet.
Management: Anticoagulation, confirm with CTPA, O₂ supplementation.
High AG Lactic Acidosis
ABG: pH 7.22, PaCO₂ 28, HCO₃⁻ 11, Lactate 6.8, AG 24
Analysis: Metabolic acidosis (lactic). Appropriate respiratory compensation (Winters: expected 24.5). Lactate = tissue hypoperfusion.
Management: Antibiotics, source control, fluid resuscitation 30 mL/kg, vasopressors if MAP <65.
High AG Metabolic Acidosis (Uremia)
ABG: pH 7.29, PaCO₂ 32, HCO₃⁻ 15, AG 18
Analysis: Metabolic acidosis from uremic acid retention. Appropriate Kussmaul breathing (compensation). AG raised from phosphates, sulfates.
Management: Oral NaHCO₃, dialysis if severe.
Metabolic Alkalosis + Hypokalemia
ABG: pH 7.56, PaCO₂ 48, HCO₃⁻ 40, K⁺ 2.8
Analysis: Metabolic alkalosis. Compensation: expected CO₂ = 0.7×(40−24)+40 = 51 ± 5 → actual 48, appropriate. Hypokalemia perpetuates alkalosis.
Management: IV NS + KCl, correct K⁺ first, treat underlying cause.
Acute Respiratory Acidosis, Normal A-a Gradient
ABG: pH 7.22, PaCO₂ 75, HCO₃⁻ 28, PaO₂ 58
Analysis: Acute respiratory acidosis. Expected HCO₃⁻ (acute): 24 + (75−40)/10 = 27.5 → appropriate. A-a gradient near normal (hypoventilation, not lung disease).
Management: Naloxone, supplemental O₂, bag-valve-mask if needed, airway protection.
Mixed Metabolic Acidosis + Respiratory Alkalosis
ABG: pH 7.38, PaCO₂ 30, HCO₃⁻ 17, AG 20
Analysis: "Normal" pH conceals dangerous double disorder. TCA → metabolic acidosis. Tachypnea → respiratory alkalosis. Both oppose each other = normal pH. Highly dangerous.
Management: IV NaHCO₃ (even to pH 7.50 to reduce QRS widening), avoid physostigmine, continuous ECG.
Respiratory Alkalosis (Compensated)
ABG: pH 7.47, PaCO₂ 30, HCO₃⁻ 21
Analysis: Hyperventilation at altitude → respiratory alkalosis → kidney compensates by excreting HCO₃⁻ (chronically). HCO₃⁻ 21 = expected chronic reduction.
Management: Acclimatization, acetazolamide if symptomatic.
Acute Respiratory Alkalosis
ABG: pH 7.55, PaCO₂ 25, HCO₃⁻ 22, PaO₂ 112
Analysis: Respiratory alkalosis. HCO₃⁻ still normal → acute (no renal response yet). PaO₂ high because of hyperventilation. A-a gradient normal.
Management: Reassurance, controlled breathing, treat underlying anxiety.
Combined Respiratory & Metabolic Acidosis
ABG: pH 7.18, PaCO₂ 60, HCO₃⁻ 21
Analysis: Mixed acidosis. HCO₃⁻ should be higher if this were purely respiratory acidosis (expected ≈ 27 chronic). HCO₃⁻ 21 means metabolic acidosis is also present (lactic from sepsis or hypoperfusion).
Management: Intubation + ventilation, antibiotics, source control.
Post-Hypercapnic Alkalosis
ABG: pH 7.57, PaCO₂ 40, HCO₃⁻ 36
Analysis: Patient was chronic CO₂ retainer (COPD). PaCO₂ over-corrected to "normal" by ventilator. Elevated HCO₃⁻ remains — metabolic alkalosis revealed.
Management: Allow gentle permissive hypercapnia (target PaCO₂ 50–60), don't force normalization.
NAGMA + Elevated AG
ABG: pH 7.24, PaCO₂ 28, HCO₃⁻ 12, Na 138, Cl 114, AG 12
Analysis: Normal AG metabolic acidosis from bicarbonate loss in diarrhea. PLUS hyperchloremic component from saline-loaded kidneys. Winter's confirms appropriate compensation. No additional disorders.
Management: Treat diarrhea cause, fluid/electrolyte replacement.
Mixed Respiratory Alkalosis + Metabolic Acidosis
ABG: pH 7.39, PaCO₂ 26, HCO₃⁻ 15, Lactate 4.1
Analysis: Liver failure → lactic acidosis (reduced lactate clearance) + respiratory alkalosis (ammonia-driven hyperventilation). pH "normal" but two opposing disorders. Lactate elevated confirms acidosis component.
Management: Treat encephalopathy, transplant evaluation, lactulose, rifaximin.
Top Clinical Pearls
- Never stop at pH. A pH of 7.40 can hide a DKA with vomiting, TCA overdose, or salicylate poisoning.
- Winter's Formula is your best friend in metabolic acidosis — always verify if compensation is appropriate.
- Correct for albumin before calling AG normal — low albumin masks the gap.
- A-a gradient separates pure hypoventilation from lung disease. Normal A-a = central cause.
- Delta-delta is only useful in high AG metabolic acidosis. Always ask: is the HCO₃⁻ lower or higher than expected for this AG?
- In COPD, target SpO₂ 88–92% — not 98%. Over-oxygenation blunts hypoxic drive and worsens CO₂ retention.
- Salicylate toxicity: never be reassured by near-normal pH — it can rapidly decompensate.
- Bicarbonate in DKA: only if pH <6.9. Routine HCO₃⁻ worsens hypokalemia and paradoxical CNS acidosis.