ABG Interpretation Step by Step: ROME Mnemonic and Tic-Tac-Toe Method for Nursing Students

Step 1: Look at pH — is it acidotic (<7.35) or alkalotic (>7.45)? Step 2: Look at PaCO2 (respiratory parameter) — is it elevated (>45) or low (<35)? Step 3: Look at HCO3 (metabolic parameter) — is it elevated (>26) or low (<22)? Step 4: Decide which parameter (CO2 or HCO3) matches the pH direction — that is the PRIMARY disturbance. ROME: Respiratory Opposite (CO2 moves opposite to pH in respiratory disorders), Metabolic Equal (HCO3 moves same direction as pH in metabolic). Step 5: Assess compensation — is the other parameter moving to offset (compensation present, partial or full) or unchanged (uncompensated)? Normal ranges: pH 7.35-7.45, PaCO2 35-45 mmHg, HCO3 22-26 mEq/L, PaO2 80-100 mmHg.

RESPIRATORY OPPOSITE: in respiratory acid-base disorders, the pH and PaCO2 move in OPPOSITE directions. Low pH + high PaCO2 = respiratory acidosis (e.g., COPD exacerbation, hypoventilation). High pH + low PaCO2 = respiratory alkalosis (e.g., anxiety, hyperventilation, salicylate toxicity early). METABOLIC EQUAL: in metabolic disorders, pH and HCO3 move in the SAME direction. Low pH + low HCO3 = metabolic acidosis (e.g., DKA, lactic acidosis, renal failure). High pH + high HCO3 = metabolic alkalosis (e.g., vomiting, diuretic use, hypokalemia). The mnemonic is the fastest way to identify the primary disturbance from the three numbers.

Draw a 3×3 grid. Label rows: Acidotic / Normal / Alkalotic. Label columns: pH / PaCO2 / HCO3. Place each value in the row matching whether it is low, normal, or high (using the disorder direction — high PaCO2 is acidotic; low PaCO2 is alkalotic; low HCO3 is acidotic; high HCO3 is alkalotic). The PRIMARY disturbance is the row containing the pH; the COMPENSATION is in the opposite row. If both PaCO2 and HCO3 are in the same row as pH = uncompensated. If they are in OPPOSITE rows = compensation present (partial if pH abnormal, full if pH normal). This visual method is heavily taught in nursing programs for its bedside speed and reliability.

A 68-year-old with COPD: pH 7.30, PaCO2 60 mmHg, HCO3 30 mEq/L. Step 1: pH 7.30 is acidotic. Step 2: PaCO2 60 is elevated (acidotic — respiratory primary). Step 3: HCO3 30 is elevated (alkalotic — opposite direction from pH, so this is compensation). Step 4: Primary = respiratory acidosis. Step 5: HCO3 elevated but pH still acidotic = PARTIAL metabolic compensation. Interpretation: partially compensated respiratory acidosis from CO2 retention. Clinical correlation: chronic CO2 retention in COPD with kidneys raising HCO3 over weeks to partially compensate.

A 22-year-old type 1 diabetic: pH 7.20, PaCO2 22 mmHg, HCO3 10 mEq/L. Step 1: pH 7.20 is acidotic. Step 2: PaCO2 22 is low (alkalotic — opposite direction, so compensation). Step 3: HCO3 10 is low (acidotic — same direction as pH, so primary). Step 4: Primary = metabolic acidosis. Step 5: PaCO2 low (Kussmaul breathing) but pH still acidotic = PARTIAL respiratory compensation. Clinical: DKA with anion-gap metabolic acidosis from ketoacids; the body hyperventilates (Kussmaul respirations) to blow off CO2 and partially compensate. Calculating the anion gap (Na - Cl - HCO3) confirms the gap acidosis pattern characteristic of DKA, lactic acidosis, and salicylate toxicity.

A 28-year-old with acute anxiety: pH 7.52, PaCO2 28 mmHg, HCO3 23 mEq/L. Step 1: pH 7.52 is alkalotic. Step 2: PaCO2 28 is low (alkalotic — same direction as pH, but ROME says respiratory is opposite... wait — pH alkalotic AND PaCO2 low means PaCO2 went OPPOSITE to what would cause alkalosis if it were respiratory? Re-check: respiratory alkalosis is caused by EXCESSIVE CO2 elimination (hyperventilation), which makes PaCO2 LOW and pH HIGH. So low PaCO2 → high pH is the SAME direction interpretation for whether it explains the disturbance, but ROME notation calls this OPPOSITE because the numbers move in opposite directions on a chart. Apply correctly: pH high (alkalotic), PaCO2 low (alkalotic when considered as a respiratory cause). HCO3 23 (normal). Primary = respiratory alkalosis. Compensation = none (HCO3 normal). Acute hyperventilation has not had time for renal HCO3 compensation.

Snap a photo of any ABG result and NurseIQ identifies the primary disturbance using ROME, draws the tic-tac-toe grid, assesses compensation, and provides a likely clinical scenario matching the values. The app generates NCLEX-style practice questions at three difficulty levels and tracks your ABG accuracy across patterns (respiratory acidosis, metabolic acidosis, mixed disorders).