🌬️ Lab Values & Diagnostics Lesson

ABG: reading acid-base status in four numbers

The ABG is one of the most feared lab interpretations in nursing and medical training — but a consistent 4-step process makes it manageable every time.

7.35-7.45

CO2

35-45

HCO3

22-26

PaO2

80-100

📖 Full Breakdown

A 4-step interpretation process, using the ROME shortcut

ROME (Respiratory Opposite, Metabolic Equal) tells you which direction CO2 or HCO3 should move relative to pH in an uncompensated primary disorder.

Step 1 — Check pH

7.35–7.45 is normal

Below 7.35 is acidosis; above 7.45 is alkalosis. This tells you the overall direction of the problem before you determine the cause.

Step 2 — Check PaCO2

35–45 mmHg — the respiratory component

If PaCO2 is abnormal in a direction that explains the pH change, the primary problem is respiratory.

Step 3 — Check HCO3

22–26 mEq/L — the metabolic component

If HCO3 is abnormal in a direction that explains the pH change, the primary problem is metabolic.

Step 4 — Apply ROME

Respiratory Opposite, Metabolic Equal

In a respiratory disorder, pH and CO2 move in OPPOSITE directions (high CO2 = low pH). In a metabolic disorder, pH and HCO3 move in the SAME direction (high HCO3 = high pH).

🩺 Clinical / Exam Application

A COPD patient's ABG shows pH 7.30 (acidosis) and PaCO2 60 (elevated). Applying ROME: respiratory disorders move pH and CO2 in opposite directions — here, low pH pairs with high CO2, confirming this is respiratory acidosis, consistent with the patient's chronic lung disease causing CO2 retention. This same 4-step process works identically whether the underlying cause is COPD, DKA, vomiting, or hyperventilation — only the specific numbers change.

⚠️ Exam Alert

A frequently tested scenario pairs each of the four acid-base disorders with a classic clinical cause: respiratory acidosis (COPD, opioid overdose), metabolic acidosis (DKA, lactic acidosis), respiratory alkalosis (hyperventilation/anxiety), and metabolic alkalosis (vomiting, NG suction) — know these pairings, since exam questions often present the clinical scenario and ask you to identify the acid-base pattern.

🚧 Common Trap

Don't assume abnormal CO2 or HCO3 alone identifies the primary disorder without checking against the pH direction. A compensating value can also be abnormal — the primary disorder is identified by which abnormal value MATCHES the direction of the pH change per ROME, not simply which value happens to be out of range.

✅ Quick Check

A patient's ABG shows pH 7.50 and HCO3 32. Using ROME, is this primarily respiratory or metabolic, and is it acidosis or alkalosis?

📝 Exam Prep

Common Exam Questions

❓ How do you interpret an ABG and what are the normal values?

✅ Normal values: pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3 22–26 mEq/L, PaO2 80–100 mmHg. Interpretation steps: (1) check pH for acidosis/alkalosis, (2) check CO2 for a respiratory cause, (3) check HCO3 for a metabolic cause, (4) assess for compensation.

❓ What does the ROME method stand for in ABG interpretation?

✅ ROME = Respiratory Opposite, Metabolic Equal. In respiratory disorders, pH and CO2 move in opposite directions. In metabolic disorders, pH and HCO3 move in the same direction.