ABG Interpretation for the NCLEX
Updated October 27, 2026
Arterial blood gas (ABG) questions give you a pH, a PaCO2 and a bicarbonate level, and ask what’s going on. With three normal ranges and one memory trick, you can work out most of them in seconds. Here’s how, followed by free practice questions.
Normal values (typical adult)
| Value | Normal |
|---|---|
| pH | 7.35 to 7.45 |
| PaCO2 | 35 to 45 mm Hg |
| HCO3 (bicarbonate) | 22 to 26 mEq/L |
Three steps
- Look at the pH. Below 7.35 is acidosis; above 7.45 is alkalosis.
- Find the cause. PaCO2 is the respiratory part (CO2 acts as an acid). HCO3 is the metabolic part (a base).
- Check for compensation. If the other value has moved to push the pH back toward normal, the body is compensating. If the pH is back in the normal range, it’s fully compensated.
The ROME trick
Respiratory Opposite: in respiratory problems, pH and PaCO2 move in opposite directions (pH down, CO2 up = respiratory acidosis).
Metabolic Equal: in metabolic problems, pH and HCO3 move in the same direction (pH down, HCO3 down = metabolic acidosis).
Common causes
| Imbalance | Common causes |
|---|---|
| Respiratory acidosis | Hypoventilation: COPD, opioid overdose, sedation |
| Respiratory alkalosis | Hyperventilation: anxiety, pain, early sepsis |
| Metabolic acidosis | Diabetic ketoacidosis, kidney failure, severe diarrhea |
| Metabolic alkalosis | Vomiting, gastric suction, too much antacid |
Free ABG practice questions
-
ABG results: pH 7.30, PaCO2 52 mm Hg, HCO3 24 mEq/L. What is the interpretation?Answer: AThe pH is low (acidosis), and PaCO2 is high, moving opposite to pH, so it's respiratory. HCO3 is normal, so there's no compensation yet.
-
ABG results: pH 7.50, PaCO2 40 mm Hg, HCO3 32 mEq/L. What is the interpretation?Answer: CThe pH is high (alkalosis), and HCO3 is high, moving in the same direction as pH, so it's metabolic. Vomiting and gastric suction are common causes.
-
A client is breathing rapidly during a panic attack. Which imbalance is most likely?Answer: DHyperventilation blows off CO2, lowering PaCO2 and raising the pH, which causes respiratory alkalosis.
-
A client with diabetic ketoacidosis would most likely have which ABG pattern?Answer: ADiabetic ketoacidosis causes metabolic acidosis: a low pH with a low bicarbonate. Breathing may become deep and rapid to blow off CO2.
-
ABG results: pH 7.38, PaCO2 55 mm Hg, HCO3 32 mEq/L in a client with long-standing COPD. What is the interpretation?Answer: BThe pH is normal but on the acidic side, PaCO2 is high (respiratory acidosis) and HCO3 is high (the kidneys compensating). With the pH back in range, it's fully compensated.
-
Which value is the respiratory part of an ABG?Answer: CPaCO2 reflects breathing: the lungs control how much CO2 is kept or blown off. HCO3 is the metabolic part, controlled by the kidneys.
Pass the NCLEX-RN on your first try
- All 178 questions across the eight Client Needs categories of the 2026 NCLEX-RN test plan
- An explanation for every answer, with the guideline, drug label or priority rule behind it
- Unlimited full 85-question practice exams, with each area in its test plan share