Reference — ABG & Acid-Base
Normal ABG Values
Reference ranges for arterial blood gases, the venous values they are often confused with, and the thresholds that grade hypoxemia — the baseline numbers every interpretation starts from.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
The ranges below are typical adult values at sea level breathing room air. Individual laboratories publish their own reference ranges, and several values shift with age, altitude, and chronic disease. Always read oxygenation (PaO₂ and SaO₂) separately from the acid-base values, and compare every gas to the patient’s own baseline rather than the textbook number alone.
Arterial Reference Ranges
| Value | Normal Range | What It Reflects |
|---|---|---|
| pH | 7.35 – 7.45 | Net acid-base balance |
| PaCO₂ | 35 – 45 mmHg | Respiratory component (ventilation) |
| PaO₂ | 80 – 100 mmHg (room air) | Oxygenation — read separately |
| HCO₃⁻ | 22 – 26 mEq/L | Metabolic component (renal) |
| Base excess | −2 to +2 mEq/L | Metabolic acid-base deviation |
| SaO₂ | 95 – 100% | Hemoglobin oxygen saturation |
Venous Comparison
| Parameter | Venous (typical) | Arterial (for contrast) |
|---|---|---|
| pH | 7.31 – 7.41 | 7.35 – 7.45 |
| PCO₂ | 41 – 51 mmHg | 35 – 45 mmHg |
| PO₂ | 35 – 45 mmHg | 80 – 100 mmHg |
Never interpret venous values against arterial norms. Venous pH runs lower and venous PCO₂ higher than arterial, and venous PO₂ says nothing about arterial oxygenation. A venous sample read as if it were arterial manufactures a respiratory acidosis and a hypoxemia that the patient does not have. Confirm the sample source before interpreting.
Hypoxemia Severity
| Severity | PaO₂ (room air) |
|---|---|
| Mild | PaO₂ 60 – 79 mmHg |
| Moderate | PaO₂ 40 – 59 mmHg |
| Severe | PaO₂ < 40 mmHg |
Clinical Notes
- Age lowers expected PaO₂. Arterial oxygen tension declines gradually with age. A rough bedside approximation for older adults is expected PaO₂ ≈ 100 − (age ÷ 3). Treat it as an estimate of what is reasonable for the patient, not a value to chart.
- Chronic CO₂ retainers run their own baseline. Patients with long-standing hypercapnia (e.g., COPD) carry an elevated PaCO₂ with a near-normal pH from renal compensation. Their “acceptable” range is the patient’s own compensated baseline, not the textbook 35 – 45 mmHg.
- Capillary samples approximate pH and PCO₂ only. Arterialized capillary blood tracks arterial pH and PCO₂ reasonably well but does not reliably reflect PaO₂ — never read oxygenation from a capillary gas.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Analysis and monitoring of gas exchange.
- Malley WJ. Clinical Blood Gases: Assessment and Intervention. 2nd ed. Elsevier Saunders; 2005.
- Davis MD, Walsh BK, Sittig SE, Restrepo RD. AARC Clinical Practice Guideline: Blood gas analysis and hemoximetry: 2013. Respir Care. 2013;58(10):1694-1703.