ABG interpretation comes down to 4 questions. Learn the steps, then check your answers with the free calculator below.
Normal ABG values
| pH | 7.35-7.45 |
| PaCO2 | 35-45 mmHg (respiratory, acid) |
| HCO3 | 22-26 mEq/L (metabolic, base) |
| PaO2 | 80-100 mmHg |
| SaO2 | 95-100% |
ABG interpretation in 4 steps
- Look at pH. Below 7.35 is acidosis. Above 7.45 is alkalosis.
- Look at PaCO2. Does it explain the pH? High CO2 causes acidosis; low CO2 causes alkalosis. If yes, it is respiratory.
- Look at HCO3. Does it explain the pH? Low HCO3 causes acidosis; high HCO3 causes alkalosis. If yes, it is metabolic.
- Check compensation.
- Other value normal: uncompensated.
- Other value abnormal in the direction that pushes pH back toward normal (HCO3 high in respiratory acidosis, PaCO2 low in metabolic acidosis), pH still abnormal: partially compensated.
- Same, but pH back in the normal range: fully compensated.
- Respiratory Opposite: pH and PaCO2 move opposite ways.
- Metabolic Equal: pH and HCO3 move the same way.
Free ABG interpretation calculator
ABG interpretation chart
| Disorder | pH | PaCO2 | HCO3 | Common causes |
|---|---|---|---|---|
| Respiratory acidosis | ↓ | ↑ | Normal or ↑ (compensating) | COPD, opioid overdose, hypoventilation, sedation |
| Respiratory alkalosis | ↑ | ↓ | Normal or ↓ (compensating) | Anxiety, pain, hyperventilation, early PE |
| Metabolic acidosis | ↓ | Normal or ↓ (compensating) | ↓ | DKA, diarrhea, kidney failure, lactic acidosis |
| Metabolic alkalosis | ↑ | Normal or ↑ (compensating) | ↑ | Vomiting, NG suction, loop or thiazide diuretics |
What the NCLEX asks about ABGs
- Interpret the gas, then link it to the cause in the stem (vomiting, DKA, COPD, anxiety).
- Metabolic acidosis often shifts potassium out of cells, so watch for hyperkalemia.
- Kussmaul respirations are the lungs blowing off CO2 to compensate for metabolic acidosis.
- Anxious hyperventilation causes respiratory alkalosis. Coaching slower breathing is the nursing action.
- Many clients with COPD live with a compensated respiratory acidosis at baseline.
ABG interpretation practice
A client who received IV opioids is drowsy. ABG: pH 7.30, PaCO2 58 mmHg, HCO3 24 mEq/L. How should the nurse interpret this?
Rationale: pH is low (acidosis) and PaCO2 is high, so the cause is respiratory. HCO3 is normal, so the kidneys have not compensated yet. Hypoventilation from opioids retains CO2.
An anxious client is breathing rapidly. ABG: pH 7.48, PaCO2 30 mmHg, HCO3 23 mEq/L. Which interpretation is correct?
Rationale: pH is high (alkalosis) and PaCO2 is low from blowing off CO2, so it is respiratory. HCO3 is normal, so it is uncompensated. Coach slow breathing.
A client with diabetic ketoacidosis has deep, rapid respirations. ABG: pH 7.29, PaCO2 30 mmHg, HCO3 15 mEq/L. Which interpretation is correct?
Rationale: pH is low and HCO3 is low, so it is metabolic acidosis. PaCO2 is low because the lungs are blowing off CO2 to compensate (Kussmaul respirations), but pH is still abnormal, so it is partially compensated.
A client has had NG suction for 3 days. ABG: pH 7.50, PaCO2 48 mmHg, HCO3 34 mEq/L. Which interpretation is correct?
Rationale: pH is high and HCO3 is high, so it is metabolic alkalosis from loss of gastric acid. PaCO2 is elevated as the lungs retain CO2 to compensate, but pH is still abnormal, so it is partially compensated.
A client with long-standing COPD has this ABG: pH 7.37, PaCO2 52 mmHg, HCO3 30 mEq/L. Which interpretation is correct?
Rationale: pH is normal but on the acidic side of 7.40. PaCO2 is high (respiratory acidosis) and HCO3 is high because the kidneys have retained bicarbonate. With pH back in range, it is fully compensated, a common baseline in COPD.
More practice: free NCLEX practice questions. Normal ranges: NCLEX lab values chart.
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Frequently asked questions
What are normal ABG values?
pH 7.35 to 7.45, PaCO2 35 to 45 mmHg, HCO3 22 to 26 mEq/L, PaO2 80 to 100 mmHg, and SaO2 95 to 100 percent.
What does ROME stand for in ABGs?
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.
How do you tell if an ABG is compensated?
If the value not causing the problem has moved in the direction that pushes pH back toward normal (for example, HCO3 high in respiratory acidosis), the body is compensating. If pH is still abnormal, it is partially compensated. If pH is back in the normal range, it is fully compensated.