Board Prep Shortcut
pH and PaCO₂ move opposite directions in respiratory disorders. pH and HCO₃ move the same direction in metabolic disorders.
How to Interpret an ABG
Arterial blood gas interpretation is one of the most important skills for respiratory therapists. Whether you are preparing for clinicals, studying for the TMC, or reviewing for the CSE, this guide walks you through a systematic process for interpreting any ABG.
Use the Same ABG Process Every Time
Many students try to interpret ABGs by memorizing isolated patterns. Experienced respiratory therapists use a repeatable sequence that reduces guessing and helps connect the numbers to the patient.
1Evaluate pH
2Find the cause
3Assess compensation
4Assess oxygenation
5Build interpretation
Evaluate pH
Start by deciding whether the blood is acidotic, alkalotic, or normal. Do this before looking at PaCO₂ or HCO₃.
| pH Value | Interpretation |
|---|---|
| Less than 7.35 | Acidosis / acidemia |
| 7.35–7.45 | Normal pH range |
| Greater than 7.45 | Alkalosis / alkalemia |
Example: pH 7.28 is acidotic because it is below 7.35.
Determine Respiratory or Metabolic Cause
After you know the pH direction, compare it to PaCO₂ and HCO₃. The value that matches the pH problem identifies the primary disorder.
Normal PaCO₂:
35–45 mmHg
35–45 mmHg
Normal HCO₃:
22–26 mEq/L
22–26 mEq/L
| Pattern | How to Recognize It | Example |
|---|---|---|
| Respiratory acidosis | pH low and PaCO₂ high | pH 7.28, PaCO₂ 60 |
| Respiratory alkalosis | pH high and PaCO₂ low | pH 7.50, PaCO₂ 28 |
| Metabolic acidosis | pH low and HCO₃ low | pH 7.25, HCO₃ 16 |
| Metabolic alkalosis | pH high and HCO₃ high | pH 7.49, HCO₃ 34 |
Assess Compensation
Compensation occurs when the body attempts to bring the pH back toward normal. The lungs compensate for metabolic disorders, and the kidneys compensate for respiratory disorders.
No compensation
The opposing system remains normal.
The opposing system remains normal.
Partial compensation
Both systems are abnormal, but pH is still abnormal.
Both systems are abnormal, but pH is still abnormal.
Full compensation
Both systems are abnormal, and pH has returned to the normal range.
Both systems are abnormal, and pH has returned to the normal range.
| Example | Interpretation | Why |
|---|---|---|
| pH 7.28, PaCO₂ 60, HCO₃ 24 | Respiratory acidosis with no compensation | HCO₃ is still normal. |
| pH 7.32, PaCO₂ 58, HCO₃ 30 | Partially compensated respiratory acidosis | HCO₃ is elevated, but pH remains acidotic. |
| pH 7.38, PaCO₂ 56, HCO₃ 32 | Fully compensated respiratory acidosis | Both PaCO₂ and HCO₃ are abnormal, but pH is back in range. |
Evaluate Oxygenation
Acid-base status and oxygenation are related, but they are not the same thing. After interpreting pH, PaCO₂, and HCO₃, assess PaO₂ separately.
| PaO₂ | Oxygenation Interpretation |
|---|---|
| 80–100 mmHg | Normal oxygenation |
| 60–79 mmHg | Mild hypoxemia |
| 40–59 mmHg | Moderate hypoxemia |
| Less than 40 mmHg | Severe hypoxemia |
Example: PaO₂ 55 mmHg is moderate hypoxemia.
Build the Final ABG Interpretation
Once each step is complete, combine the findings into one complete interpretation.
pH7.32
PaCO₂58
HCO₃30
PaO₂62
pH: Acidotic
Cause: Respiratory
Compensation: Partial
Oxygenation: Mild hypoxemia
Complete Interpretation
Partially compensated respiratory acidosis with mild hypoxemia.
What to Avoid When Reading ABGs
Starting with PaCO₂: Always classify pH first so you know which direction the problem is moving.
Ignoring compensation: Compensation helps you decide whether the problem is acute, partially corrected, or fully compensated.
Forgetting oxygenation: A patient can have a clear acid-base disorder and still need a separate oxygenation assessment.
Guessing from one value: Use the same five-step method every time.
Build Respiratory Therapy Confidence
PulmoLearn includes interactive respiratory therapy lessons, disease modules, clinical reasoning activities, ABG review, oxygen therapy, pulmonary assessment, mechanical ventilation, and board preparation.