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ABG Practice Questions: Interpret Arterial Blood Gases Step by Step
Arterial blood gas questions look intimidating, but every one is solved with the same three steps: look at the pH, find which value matches it, then check compensation.
Normal ranges used below: pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, HCO3 22 to 26 mEq/L.
What ABG questions test
- Identifying acidosis versus alkalosis
- Respiratory versus metabolic cause
- Uncompensated, partially and fully compensated results
- Conditions that cause each imbalance
6 abg interpretation practice questions with rationales
Answer each one before opening the rationale. Every option is explained, including the wrong ones.
- Uncompensated respiratory acidosis
- Uncompensated metabolic acidosis
- Compensated respiratory alkalosis
- Uncompensated metabolic alkalosis
Show answer and rationale
Answer: A
A. Correct. pH is low (acidosis), PaCO2 is high and matches, HCO3 is normal so there is no compensation.
B. HCO3 is normal, so the cause is not metabolic.
C. The pH is acidotic, not alkalotic.
D. The pH is acidotic.
- Respiratory acidosis
- Uncompensated respiratory alkalosis
- Metabolic alkalosis
- Normal ABG
Show answer and rationale
Answer: B
A. pH is high, so this is alkalosis.
B. Correct. High pH with low PaCO2 (CO2 blown off) and normal HCO3.
C. HCO3 is normal, so the cause is respiratory.
D. pH and PaCO2 are both outside normal.
- Uncompensated metabolic acidosis
- Respiratory acidosis
- Metabolic alkalosis
- Fully compensated metabolic acidosis
Show answer and rationale
Answer: A
A. Correct. Low pH, low HCO3 matches, PaCO2 normal so no compensation yet.
B. PaCO2 is normal, so the cause is not respiratory.
C. pH is acidotic.
D. The pH is still abnormal, so it is not fully compensated.
- Uncompensated respiratory acidosis
- Fully compensated respiratory acidosis
- Partially compensated metabolic alkalosis
- Normal ABG
Show answer and rationale
Answer: B
A. The pH is within normal, so it is compensated.
B. Correct. pH is normal but on the acid side of 7.40; high PaCO2 is the cause and high HCO3 is the kidneys' compensation. Typical of chronic COPD.
C. The pH leans acidic, so the primary problem is acidosis.
D. PaCO2 and HCO3 are both abnormal.
- Severe diarrhea
- Prolonged vomiting or NG suction
- Diabetic ketoacidosis
- Opioid overdose
Show answer and rationale
Answer: B
A. Diarrhea loses bicarbonate and causes metabolic acidosis.
B. Correct. Losing stomach acid leaves excess base.
C. Ketones cause metabolic acidosis.
D. Slowed breathing causes respiratory acidosis.
- COPD exacerbation
- Opioid overdose
- Anxiety with hyperventilation
- Guillain-Barré syndrome affecting the respiratory muscles
- Prolonged vomiting
Show answer and rationale
Answer: A, B, D
A. Correct. Air trapping retains CO2.
B. Correct. Respiratory depression retains CO2.
C. Hyperventilation blows off CO2 and causes respiratory alkalosis.
D. Correct. Weak respiratory muscles cause hypoventilation.
E. Vomiting causes metabolic alkalosis.
How to interpret any ABG in three steps
- Step 1: pH below 7.35 is acidosis, above 7.45 is alkalosis.
- Step 2: Find the value that matches. CO2 acts as an acid (high CO2 means acidosis); HCO3 acts as a base (low HCO3 means acidosis). A CO2 match is respiratory, an HCO3 match is metabolic.
- Step 3: Check the other value. Normal means uncompensated. Abnormal in the opposite direction with pH still out of range means partially compensated. pH back in range means fully compensated.
- ROME: Respiratory Opposite (pH and CO2 move in opposite directions), Metabolic Equal (pH and HCO3 move in the same direction).
Frequently asked questions
pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, HCO3 22 to 26 mEq/L, and PaO2 about 80 to 100 mmHg.
Respiratory Opposite, Metabolic Equal: in respiratory problems pH and PaCO2 move in opposite directions; in metabolic problems pH and HCO3 move in the same direction.
If the value not causing the problem has moved out of range in the opposite direction, the body is compensating. If the pH is back to normal, it is fully compensated.