SpO2 Oxygen Saturation Guide
Enter your pulse oximeter SpO2 reading to see whether it is normal, concerning, or an emergency.
Covers all saturation ranges from 100% down to critical.
Blood oxygen saturation (SpO₂) is the percentage of hemoglobin molecules in the blood that are bound to oxygen. It is measured non-invasively using a pulse oximeter, which shines red and infrared light through a fingertip and measures the differential absorption by oxygenated vs. deoxygenated hemoglobin.
Pulse oximeter measurement principle: SpO₂ (%) = (HbO₂ ÷ (HbO₂ + Hb)) × 100
Where:
- HbO₂: oxyhemoglobin (hemoglobin bound to oxygen)
- Hb: deoxyhemoglobin (hemoglobin not carrying oxygen)
The device calculates the ratio using Beer-Lambert law absorption at two wavelengths:
- Red light (~660 nm): Deoxyhemoglobin absorbs more
- Infrared light (~940 nm): Oxyhemoglobin absorbs more
The R-ratio (internal calculation): R = (AC₆₆₀/DC₆₆₀) ÷ (AC₉₄₀/DC₉₄₀)
The device maps R to SpO₂ via empirically derived calibration curves.
SpO₂ reference ranges:
| SpO₂ Value | Interpretation | Action |
|---|---|---|
| 95–100% | Normal | No action needed |
| 91–94% | Mild hypoxemia | Monitor; seek medical advice |
| 86–90% | Moderate hypoxemia | Medical evaluation needed |
| Below 86% | Severe hypoxemia | Emergency — seek immediate care |
| Below 80% | Critical | Life-threatening |
Normal values by context:
- Healthy adult at sea level: 96–99%
- Healthy adult at altitude (3,000m/10,000 ft): 90–94% is normal due to lower partial pressure of O₂
- During sleep (healthy): may dip briefly to 92–94% (normal)
- Sleep apnea: may drop to 80–88% during apnea events (abnormal)
- COPD patients: baseline of 88–92% may be their acceptable baseline (consult physician)
Factors that affect pulse oximeter accuracy:
- Nail polish (especially dark blue/black): can reduce accuracy by 2–3%
- Poor peripheral circulation: cold hands, shock → lower signal quality
- Carbon monoxide poisoning: pulse oximeter reads CO-hemoglobin as oxygenated → falsely high SpO₂ (dangerously misleading, use ABG testing in suspected CO poisoning)
- Skin pigmentation: darker skin tones have shown 3× higher rate of occult hypoxemia missed by pulse oximeters (NEJM 2020 study), be aware of this limitation
- Motion artifact: even slight finger movement causes noise; hold still for accurate reading
Oxygen-hemoglobin dissociation curve: SpO₂ and partial pressure of oxygen (PaO₂) are related non-linearly:
- SpO₂ 98% ≈ PaO₂ 100 mmHg (normal)
- SpO₂ 90% ≈ PaO₂ 60 mmHg (inflection point: below this, PaO₂ drops rapidly)
- SpO₂ 75% ≈ PaO₂ 40 mmHg (mixed venous blood: severely low)
The steep portion of the curve (below 90% SpO₂) means small further drops in SpO₂ correspond to large drops in blood oxygen delivery — this is why 90% is the clinical threshold requiring intervention.
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