Systematic 6-step ABG interpretation (use this every time)

1. 1) Look at the clinical context & ABCs.
2. 2) Check pH.
3. 3) Look at PaCO₂ (respiratory) and HCO₃⁻ (metabolic).
4. 4) Decide primary process (respiratory vs metabolic) and whether compensation is appropriate.
5. 5) Calculate anion gap
6. 6) Look for mixed disorders.

Key equations & rules (copy these)

• Winter’s formula (metabolic acidosis): Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2. If measured PaCO₂ is higher than expected → mixed respiratory acidosis; if lower → additional respiratory alkalosis. 
• Anion gap (AG): AG = Na − (Cl + HCO₃). Normal AG ≈ 8–12 (depends on lab). Elevated AG → HAGMA (lactate, toxins, renal failure). 
• Delta (Δ) ratio: ΔAG / ΔHCO₃ — helps detect mixed metabolic disorders. Typical interpretation: <0.4 (pure NAGMA), 0.4–0.8 (mixed NAGMA+HAGMA), 0.8–2.0 (pure HAGMA), >2.0 (HAGMA + metabolic alkalosis). 
• Respiratory compensation estimates: Acute respiratory acidosis: HCO₃ increases ~1 mEq/L per 10 mmHg ↑PaCO₂; chronic: ~3–4 mEq/L per 10 mmHg. Acute respiratory alkalosis: HCO₃ falls ~2 mEq/L per 10 mmHg ↓PaCO₂. (Use these to detect mixed disorders.) :

Worked examples — step by step (showing arithmetic)

Example A — Metabolic acidosis & Winter’s formula

ABG: pH 7.20, PaCO₂ 40 mmHg, HCO₃⁻ 12 mmol/L. Clinical: vomiting absent; tachypnic patient.

1. Step 1 — pH 7.20 → acidemia (primary acidosis likely).
2. Step 2 — HCO₃⁻ is low (12) → metabolic acidosis is primary.
3. Step 3 — Use Winter’s formula to predict expected PaCO₂:

Expected PaCO₂ = (1.5 × HCO₃) + 8 ± 2
= (1.5 × 12) + 8 ± 2
= 18 + 8 ± 2
= 26 ± 2 → expected range ≈ 24–28 mmHg

Measured PaCO₂ is 40 mmHg — much higher than expected → there is a concurrent respiratory acidosis (hypoventilation) on top of metabolic acidosis (mixed disorder). This is clinically important (e.g., respiratory failure, sedation) because the patient is not ventilating enough to compensate. 

Example B — Anion gap & delta ratio (HAGMA analysis)

Labs: Na 140 mmol/L, Cl 100 mmol/L, HCO₃⁻ 12 mmol/L.

1. AG = Na − (Cl + HCO₃) = 140 − (100 + 12) = 140 − 112 = 28.
2. Assuming normal AG ≈ 12 → ΔAG = 28 − 12 = 16.
3. ΔHCO₃ = 24 (expected normal) − measured HCO₃ (12) = 12.
4. Delta ratio = ΔAG / ΔHCO₃ = 16 / 12 = 1.33.

Interpretation: delta ratio ≈1.33 (between 0.8 and 2.0) suggests a pure high-anion-gap metabolic acidosis (HAGMA) — e.g., lactic acidosis, ketoacidosis, or toxin. Use clinical context and investigations to pinpoint cause.

ABG patterns & common causes (concise)

• Metabolic acidosis (low pH, low HCO₃): DKA, lactic acidosis, renal failure, toxin ingestion. Check AG and lactate. :contentReference[oaicite:16]{index=16}
• Metabolic alkalosis (high pH, high HCO₃): Vomiting, diuretics, post-hypercapnia; check urine chloride. :contentReference[oaicite:17]{index=17}
• Respiratory acidosis (low pH, high PaCO₂): Hypoventilation — opioids, respiratory muscle weakness, COPD exacerbation. Acute vs chronic compensation differs. :contentReference[oaicite:18]{index=18}
• Respiratory alkalosis (high pH, low PaCO₂): Hyperventilation (sepsis, pain, anxiety), salicylates (early).

Practical bedside tips & pitfalls

• Arterial vs venous gases: venous blood gases roughly track pH & HCO₃ but PaO₂ is not useful — interpret with caution in severe illness. :contentReference[oaicite:20]{index=20}
• Always correlate with temperature and clinical context.
• Don’t over-rely on fixed rules.
• Document exact time & clinical state (ventilated? on O₂?):

Learning & exam strategy

1. Memorise the 6-step algorithm and the two key formulae (Winter’s + AG).
2. Do timed practice: interpret 5 ABGs in 10 minutes — practise both recognition and how you would act.
3. Carry the 1-page PDF flowchart on your phone or print it for ward rounds (download link at top of post).
4. Integrate ABG interpretation with clinical scenarios: ventilator adjustments, DKA protocols, sepsis resuscitation.