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How to read an arterial blood gas

Other platforms explain it with text and video. In VMS you practice it on the patient.

MD / DOPANursing (NCLEX)
Practice it in VR with Blocky

Blood gases look complex but always follow four steps. Look at the numbers in the same order every time.

Step by step

  1. Know the normal values

    pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, HCO3 22 to 26 mEq/L, PaO2 80 to 100 mmHg. [1]

  2. Step 1: pH

    Below 7.35 is acidosis, above 7.45 is alkalosis. [1]

  3. Step 2: which one explains it?

    If the CO2 moves in the opposite direction to the pH (high CO2, low pH), it is respiratory. If the HCO3 moves in the same direction as the pH, it is metabolic. [1]

  4. Step 3: compensation

    Is the other value moving to correct the pH? If the pH is normal with both values abnormal, it is fully compensated. [1]

  5. Step 4: oxygenation

    Look at the PaO2 and the oxygen the patient is breathing. [1]

  6. Metabolic acidosis: anion gap

    Sodium minus (chloride plus bicarbonate). Normal is roughly 8 to 12; a high gap suggests ketoacidosis, lactic acidosis, kidney failure or poisoning. [2]

  7. Link to the patient

    Match the result with the story and the exam. [1]

Common errors

Practice it in VMS

In VMS you do this on the patient, not on paper: every step goes on the checklist with a score, and in Teach mode Blocky tells you what comes next.

Sources

  1. AAMC Core Entrustable Professional Activities (Core EPAs)
  2. NCCPA PANCE Content Blueprint
  3. NCSBN 2026 NCLEX-RN Test Plan

Facts and steps follow AAMC Core Entrustable Professional Activities (Core EPAs); NCSBN 2026 NCLEX-RN Test Plan. VMS is not affiliated with these organizations. Updated: 2026-10-11.