ABG Compensation Decision Algorithm: Uncompensated vs Partial vs Full

Step 1: Look at the pH first — is it normal?

• pH < 7.35 → Acidosis is present → Go to Step 2
• pH > 7.45 → Alkalosis is present → Go to Step 2
• pH 7.35–7.45 → Either normal OR fully compensated → Go to Step 2A

Step 2: Identify the primary disorder — what CAUSED the pH shift?

• PaCO2 abnormal (≠ 35–45 mmHg)?
  • CO2 > 45 → Respiratory acidosis
  • CO2 < 35 → Respiratory alkalosis
• HCO3⁻ abnormal (≠ 22–26 mEq/L)?
  • HCO3⁻ < 22 → Metabolic acidosis
  • HCO3⁻ > 26 → Metabolic alkalosis
• Rule: the value that MATCHES the pH direction is the primary cause
  • pH acidotic + CO2 elevated = respiratory acidosis
  • pH acidotic + HCO3⁻ decreased = metabolic acidosis

Step 2A: pH is normal (7.35–7.45) — is this truly normal or fully compensated?

• Both CO2 and HCO3⁻ within normal range → Normal ABG, no disorder
• Both CO2 and HCO3⁻ are abnormal → Fully compensated
  • To find the primary disorder: check which side of 7.40 the pH falls
  • pH 7.35–7.39 (low-normal) → primary disorder is acidosis
  • pH 7.41–7.45 (high-normal) → primary disorder is alkalosis

Step 3: Check the OTHER system — has it responded?

• Only ONE value abnormal (CO2 or HCO3⁻, not both), pH abnormal

  • → Uncompensated — the opposite system has NOT kicked in yet
  • Example: pH 7.28, CO2 58, HCO3⁻ 24 → Uncompensated respiratory acidosis

• BOTH values abnormal, pH still abnormal

  • → Partially compensated — the opposite system is trying but hasn't corrected pH yet
  • Example: pH 7.31, CO2 55, HCO3⁻ 30 → Partially compensated respiratory acidosis

• BOTH values abnormal, pH is normal (7.35–7.45)

  • → Fully compensated — the opposite system has corrected pH back to normal range
  • Example: pH 7.37, CO2 56, HCO3⁻ 32 → Fully compensated respiratory acidosis

Quick-check summary: