ABG Interpretation — ROME & Tic-Tac-Toe

Normal ranges

Value	Normal	Direction matters
pH	7.35–7.45	<7.35 acidosis · >7.45 alkalosis
PaCO₂	35–45 mmHg	Respiratory: ↑ acid, ↓ base
HCO₃	22–26 mEq/L	Metabolic: ↓ acid, ↑ base
PaO₂	80–100 mmHg	Oxygenation, separate from acid-base

Method 1 — ROME

Respiratory Opposite, Metabolic Equal.

1. Look at pH. ↑ alkalosis, ↓ acidosis.
2. Look at CO₂. If pH and CO₂ go in opposite directions → it's respiratory.
3. Look at HCO₃. If pH and HCO₃ go in the same direction → it's metabolic.

Example: pH 7.30 (↓), CO₂ 50 (↑) → opposite → respiratory acidosis.

Example: pH 7.50 (↑), HCO₃ 30 (↑) → same → metabolic alkalosis.

Method 2 — Tic-Tac-Toe

Draw a 3×3 grid. Top row labeled Acid · Normal · Base. Left column labeled pH · CO₂ · HCO₃. Place each value in its column.

• If pH and CO₂ end up in the same column → respiratory disorder.
• If pH and HCO₃ end up in the same column → metabolic disorder.
• If both CO₂ and HCO₃ are abnormal in the matching direction → mixed disorder.
• If the "non-causing" value is shifted toward the pH side → partial compensation.
• If pH is normal but CO₂ and HCO₃ are both abnormal → fully compensated.

Compensation states

• Uncompensated — pH abnormal, only one of CO₂/HCO₃ abnormal.
• Partially compensated — pH abnormal, both CO₂ and HCO₃ abnormal.
• Fully compensated — pH back to normal range, but CO₂ and HCO₃ both abnormal.

10 practice cases

Case 1. pH 7.28, CO₂ 56, HCO₃ 24. → Uncompensated respiratory acidosis. (Think: COPD exacerbation, opioid OD.)

Case 2. pH 7.50, CO₂ 28, HCO₃ 24. → Uncompensated respiratory alkalosis. (Anxiety/hyperventilation, PE.)

Case 3. pH 7.21, CO₂ 36, HCO₃ 14. → Uncompensated metabolic acidosis. (DKA, lactic, salicylate — think MUDPILES.)

Case 4. pH 7.49, CO₂ 38, HCO₃ 32. → Uncompensated metabolic alkalosis. (Vomiting, NG suction, diuretics.)

Case 5. pH 7.32, CO₂ 52, HCO₃ 30. → Partially compensated respiratory acidosis (HCO₃ rising to compensate). Likely chronic CO₂ retainer (chronic COPD).

Case 6. pH 7.38, CO₂ 60, HCO₃ 35. → Fully compensated respiratory acidosis. (pH inside normal range, both other values shifted.)

Case 7. pH 7.28, CO₂ 30, HCO₃ 14. → Partially compensated metabolic acidosis (CO₂ dropping via Kussmaul respirations to compensate). Classic DKA pattern.

Case 8. pH 7.22, CO₂ 60, HCO₃ 16. → Mixed acidosis — both CO₂ and HCO₃ are pushing pH down. Severe — think respiratory failure with co-existing metabolic acidosis (sepsis with shock + hypoventilation).

Case 9. pH 7.55, CO₂ 26, HCO₃ 32. → Mixed alkalosis. Both CO₂ and HCO₃ are pushing pH up. Think hyperventilation + vomiting, or salicylate toxicity early phase.

Case 10. pH 7.32, CO₂ 30, HCO₃ 16, K 5.4, glucose 480, ketones large. → DKA. Partially compensated metabolic acidosis. NCLEX answer: priority intervention is fluid resuscitation, then insulin (per protocol).

Pattern recognition by clinical setting

• COPD exacerbation: respiratory acidosis with chronic compensation (HCO₃ > 28).
• DKA / lactic acidosis / salicylate / uremia: metabolic acidosis with respiratory compensation (Kussmaul).
• Vomiting / NG suction / diuretics: metabolic alkalosis.
• Anxiety / PE / sepsis early: respiratory alkalosis (hyperventilation).
• Cardiac arrest / shock / respiratory failure: often mixed — respiratory acidosis (hypoventilation) + metabolic acidosis (lactate from poor perfusion).

PaO₂ — separate from acid-base

Acid-base is the pH/CO₂/HCO₃ system. Oxygenation is a separate question — that's PaO₂ and SpO₂. A patient can have a normal pH and severe hypoxemia, or vice versa. Don't conflate them on the test.