ABGs intimidate students because there seem to be so many values and so many possible interpretations. But here's the truth: every ABG question follows the same logic, and once you have the 4-step framework, you can interpret any result in under 60 seconds.
Normal Values — Memorize These First
pH: 7.35–7.45 | PaCO₂: 35–45 mmHg | HCO₃: 22–26 mEq/L | PaO₂: 80–100 mmHg | SpO₂: 95–100%
CO₂ = acid. HCO₃ = base. CO₂ goes up → more acid → pH goes down. HCO₃ goes up → more base → pH goes up. Everything flows from here.
The 4-Step ABG Method
- 01Look at pH — acidosis or alkalosis?pH < 7.35 = acidosis. pH > 7.45 = alkalosis. This one number tells you the direction of the problem.
- 02Look at PaCO₂ — is respiratory the cause?PaCO₂ > 45 = respiratory acidosis. PaCO₂ < 35 = respiratory alkalosis. If PaCO₂ matches the pH direction → respiratory origin.
- 03Look at HCO₃ — is metabolic the cause?HCO₃ < 22 = metabolic acidosis. HCO₃ > 26 = metabolic alkalosis. If HCO₃ matches the pH direction → metabolic origin.
- 04Is compensation happening?If both PaCO₂ and HCO₃ are abnormal, the system that doesn't match pH is compensating. pH still abnormal = partial. pH back to normal = full compensation.
The ROME Memory Tool
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.
Practice ABG Questions in NurseIQ Cases
NurseIQ clinical cases include ABG interpretation as part of the full clinical picture — just like the NGN NCLEX.
Try Free →Practice Examples
Example 1: pH 7.28 · PaCO₂ 52 · HCO₃ 24
pH 7.28 → acidosis. PaCO₂ 52 (high) → matches acidosis → respiratory cause. HCO₃ 24 (normal) → no compensation. Answer: Uncompensated Respiratory Acidosis. Think: COPD, opioid overdose, hypoventilation.
Example 2: pH 7.31 · PaCO₂ 30 · HCO₃ 14
pH 7.31 → acidosis. PaCO₂ 30 (low) → opposite of acidosis → compensating. HCO₃ 14 (low) → matches acidosis → metabolic cause. Answer: Partially Compensated Metabolic Acidosis. Think: DKA, severe diarrhea.
Example 3: pH 7.48 · PaCO₂ 40 · HCO₃ 30
pH 7.48 → alkalosis. PaCO₂ 40 (normal) → no respiratory involvement. HCO₃ 30 (high) → matches alkalosis → metabolic cause. Answer: Uncompensated Metabolic Alkalosis. Think: prolonged vomiting, NG suctioning.
What to Do With the Result
Respiratory Acidosis: Improve ventilation. Position, encourage deep breathing, prepare for possible intubation.
Respiratory Alkalosis: Slow the breathing. Treat underlying anxiety or pain.
Metabolic Acidosis: Treat the cause (insulin for DKA, fluids for dehydration).
Metabolic Alkalosis: Replace fluids and electrolytes, especially chloride and potassium.
Master this 4-step method and ABG questions become one of your most confident question types. 🩸
All 8 ABG Combinations — Quick Reference
There are exactly 8 possible ABG outcomes. Once you know these, no question can surprise you.
| Condition | pH | PaCO₂ | HCO₃ | Common Causes |
|---|---|---|---|---|
| Resp. Acidosis (Uncompensated) | ↓ | ↑ | Normal | COPD, overdose, hypoventilation |
| Resp. Acidosis (Compensated) | ↓ (near normal) | ↑ | ↑ | Chronic COPD |
| Resp. Alkalosis (Uncompensated) | ↑ | ↓ | Normal | Anxiety, hyperventilation, PE |
| Resp. Alkalosis (Compensated) | ↑ (near normal) | ↓ | ↓ | Chronic hypoxia |
| Metab. Acidosis (Uncompensated) | ↓ | Normal | ↓ | DKA, renal failure, severe diarrhea |
| Metab. Acidosis (Compensated) | ↓ (near normal) | ↓ | ↓ | DKA with Kussmaul breathing |
| Metab. Alkalosis (Uncompensated) | ↑ | Normal | ↑ | Vomiting, NG suction, diuretics |
| Metab. Alkalosis (Compensated) | ↑ (near normal) | ↑ | ↑ | Prolonged vomiting |
ABG and the NCLEX — What to Expect
On the NCLEX, ABG questions almost always follow one of three patterns. Knowing these patterns before exam day means you are not problem-solving from scratch — you are recognizing a scenario you have already seen.
Pattern 1 — Interpret the result. You are given pH, PaCO₂, HCO₃ values and asked to identify the condition. Use your 4-step method. These are the most straightforward ABG questions.
Pattern 2 — Identify the priority nursing action. After interpreting the ABG, you are asked what the nurse should do first. Remember: treat the cause, not just the numbers. For respiratory acidosis, improve ventilation. For metabolic acidosis, treat the underlying condition.
Pattern 3 — Evaluate improvement. You are shown a before and after ABG and asked whether the patient is getting better. Look at whether pH is trending toward normal and whether compensation is occurring appropriately.
Common Conditions and Their ABG Patterns
The NCLEX loves to link specific clinical conditions to their ABG findings. Here are the most tested combinations:
- COPD exacerbation: Respiratory acidosis. High PaCO₂ is expected — these patients live with elevated CO₂. Watch for acute-on-chronic changes.
- Diabetic Ketoacidosis (DKA): Metabolic acidosis with respiratory compensation (Kussmaul breathing). Very low HCO₃, low pH, low PaCO₂.
- Panic attack / Anxiety: Respiratory alkalosis. Patient hyperventilates, blows off CO₂, pH rises. PaCO₂ falls below 35.
- Prolonged vomiting / NG suctioning: Metabolic alkalosis. Losing stomach acid raises the pH. HCO₃ rises, potassium often low.
- Opioid overdose: Respiratory acidosis. Opioids suppress the respiratory drive — patient hypoventilates, CO₂ builds up.
- Pulmonary embolism: Respiratory alkalosis. Patient hyperventilates due to hypoxia and anxiety. Low PaCO₂, raised pH.
When an ABG question also gives you SpO₂ or PaO₂, check oxygenation separately. A patient can have normal acid-base balance but still be dangerously hypoxic. PaO₂ below 60 mmHg or SpO₂ below 90% always requires immediate oxygen intervention — regardless of what the rest of the ABG shows.
Frequently Asked Questions
What does "uncompensated" mean on an ABG?
Uncompensated means only one body system is showing an abnormality. The pH is off, and only one of PaCO₂ or HCO₃ is out of range. The other system has not yet had time to respond. This is an acute situation.
What does "partially compensated" mean?
Both PaCO₂ and HCO₃ are abnormal, but the pH has not returned to the normal range yet. The compensating system is working but has not fully corrected the problem.
What does "fully compensated" mean?
Both PaCO₂ and HCO₃ are abnormal, but the pH is back within normal range (7.35–7.45). Note: you can still tell which system caused the original problem by which value is abnormal in the same direction as the original pH shift.
Do I need to calculate the anion gap for NCLEX?
The basic NCLEX-RN does not typically require anion gap calculation. Focus on the 4-step interpretation method and the 8 possible outcomes. The NGN format may include ABG values as part of a larger case study where you identify which findings are clinically significant.