Chart — Respiratory
Oxygenation Measurements Chart
A comparison of the three oxygenation measurements nurses use — SpO₂, PaO₂, and SaO₂ — organized by measurement source, normal range, clinical interpretation, and key limitations.
Educational use only. Oxygenation assessment requires clinical context, provider interpretation, and integration with full patient assessment. This chart supports learning and NCLEX preparation. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
Measurement Comparison
| Measure | Source | Normal Range | Interpretation |
|---|---|---|---|
| SpO₂Peripheral Oxygen Saturation | Pulse oximetry (non-invasive; photodetector at fingertip, earlobe, or forehead) | 94–100% (88–92% COPD target) | ≥94%: Normal adult (94–98% in general, 88–92% in COPD). <90%: Hypoxemia — assess immediately. <80%: Severe hypoxemia — emergency |
| PaO₂Partial Pressure of Arterial Oxygen | Arterial blood gas (ABG) — invasive; requires arterial puncture or arterial line | 80–100 mmHg (room air, adult) | >80: Normal. 60–80: Mild hypoxemia. 40–60: Moderate hypoxemia. <40: Severe hypoxemia. <60 = respiratory failure threshold |
| SaO₂Arterial Oxygen Saturation | ABG with co-oximetry — invasive; measured directly in ABG analyzer | 95–100% | ≥95%: Normal. <90%: Significant hypoxemia. Key: SaO₂ is the gold standard when SpO₂ accuracy is in doubt (CO poisoning, methemoglobinemia) |
What Each Measurement Actually Measures
SpO₂ — Peripheral Oxygen SaturationEstimated percentage of hemoglobin molecules currently bound to oxygen; inferred from light absorption difference between oxyhemoglobin and deoxyhemoglobin
Limitations: Falsely normal in CO poisoning and methemoglobinemia. Inaccurate with nail polish, poor perfusion, hypothermia, motion artifact, severe anemia
PaO₂ — Partial Pressure of Arterial OxygenDissolved oxygen pressure in arterial plasma (not bound to hemoglobin); reflects how well O₂ crosses from alveoli into bloodstream
Limitations: Invasive; single snapshot (not continuous); decreases with age (expected PaO₂ ≈ 100 − age/3); does not measure hemoglobin-bound O₂
SaO₂ — Arterial Oxygen SaturationActual percentage of hemoglobin bound to oxygen in arterial blood; co-oximetry differentiates oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin (COHgb), and methemoglobin
Limitations: Invasive; single sample; requires ABG — not continuous. More accurate than SpO₂ because co-oximetry differentiates all hemoglobin species
Clinical Action Thresholds
| Value | Clinical Meaning and Action |
|---|---|
| SpO₂ ≥94% | Normal — maintain with supplemental O₂ as needed |
| SpO₂ 88–93% | Acceptable in COPD/hypercapnic patients; hypoxemia threshold in others — assess and act |
| SpO₂ <90% | Hypoxemia — escalate O₂ delivery; notify provider |
| SpO₂ <80% | Severe hypoxemia — emergency intervention required |
| PaO₂ <60 mmHg | Respiratory failure threshold — triggers Type I respiratory failure diagnosis |
| PaO₂/FiO₂ <300 | ARDS criteria — categorized as mild, moderate, or severe |
Key Differentiators
| Feature | SpO₂ | PaO₂ | SaO₂ |
|---|---|---|---|
| Invasive? | No | Yes (arterial) | Yes (ABG co-ox) |
| Continuous? | Yes | No | No |
| CO poisoning? | Falsely normal | Near-normal (dissolved O₂ only) | Accurately detects COHgb |
| Anemia accuracy? | Can be high despite low O₂ delivery | Normal (dissolved O₂ unaffected by Hgb level) | % saturation normal; delivery still impaired |
| Primary use | Continuous bedside monitoring | Respiratory failure diagnosis; ABG | Confirm SpO₂; detect COHgb/MetHgb |
NCLEX Pearls
- ›SpO₂ is non-invasive and continuous — the primary bedside monitoring tool.
- ›PaO₂ <60 mmHg = hypoxemia and the Type I respiratory failure threshold.
- ›SpO₂ is falsely normal in CO poisoning — obtain ABG with co-oximetry when CO is suspected.
- ›SaO₂ from co-oximetry is the gold standard when SpO₂ reliability is in question.
- ›Normal SpO₂ does not guarantee adequate oxygen delivery — hemoglobin level and cardiac output also matter.
- ›SpO₂ target 88–92% for COPD; 94–98% for most other adults. Always follow the specific provider order.
Related Resources
SpO₂, PaO₂, and SaO₂ ReferenceDefinitions, normal values, oxyhemoglobin dissociation curve, and key differences
Open →Normal ABG Values ReferencepH, PaCO₂, HCO₃, PaO₂, SaO₂ — normal ranges and critical thresholds
Open →Respiratory Failure ReferenceType I vs Type II — PaO₂ thresholds, ABG patterns, and nursing implications
Open →Oxygen Delivery DevicesDevice selection, FiO₂ ranges, and nursing considerations
Open →Standards & sources
Fact-checked Jun 21, 2026This page is written to align with AARC Clinical Practice Guidelines / ABG Standards. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →