Diagnostics
Pulse oximetry: reading beyond the number
Here, 96% is the estimated oxygen saturation and 85 beats/min is the pulse rate. Neither number measures PaCO2 or establishes adequate ventilation.
Pulox Pulse Oximeter by Stefan Bellini. CC0 1.0. View larger.
A pulse oximeter uses light to estimate arterial oxygen saturation. Its SpO2 reading is useful for tracking oxygenation, but it does not describe the whole respiratory picture.
Keep the measurements separate
| Measurement | Meaning |
|---|---|
| SpO2 | Oxygen saturation estimated by a pulse oximeter. |
| SaO2 | Arterial oxygen saturation; check whether the blood-gas report measured it by co-oximetry or calculated it. |
| PaO2 | Partial pressure of oxygen in arterial blood, measured on an ABG; reported in mmHg. |
| PaCO2 | Partial pressure of carbon dioxide in arterial blood; helps assess alveolar ventilation. |
SpO2 is a percentage, not a pressure. It does not measure PaO2, PaCO2, pH, or hemoglobin concentration. A person can have a high saturation while carrying too little oxygen because hemoglobin is low.
Healthy people commonly have saturations of 95–100%. Altitude and cardiopulmonary disease affect expected values. This reference range is not an oxygen prescription or a universal treatment target.
Check the signal
Before interpreting a reading:
- Check sensor placement and follow the device instructions.
- Reduce motion and allow time for a stable signal.
- Consider cold extremities, poor circulation, and nail products.
- Compare the displayed pulse rate with the patient’s pulse. A mismatch raises concern about signal quality.
- Compare the result with the patient’s appearance, symptoms, oxygen device, and trend.
A plausible pulse rate supports signal quality; it does not eliminate every source of saturation error.
Know the blind spots
| Situation | Why the reading can mislead |
|---|---|
| Hypoventilation | SpO2 does not measure CO2. Supplemental oxygen can maintain saturation despite CO2 retention. |
| Darker skin pigmentation | Some devices may overestimate saturation and miss hypoxemia. Consider discrepancies with the broader assessment. |
| Poor perfusion or movement | The pulsatile signal may be weak or distorted. |
| Carbon monoxide exposure | A conventional two-wavelength oximeter cannot reliably distinguish carboxyhemoglobin from oxyhemoglobin. A reassuring SpO2 does not rule out poisoning. |
Blood co-oximetry measures hemoglobin species, including carboxyhemoglobin and methemoglobin. A calculated saturation on a standard blood-gas report does not replace that measurement when abnormal hemoglobins are suspected.
Worked example
A study case gives SpO2 97% on supplemental oxygen and PaCO2 60 mmHg. These results can coexist: the saturation looks reassuring, while arterial CO2 is elevated.
The SpO2 does not establish adequate ventilation. Interpret the PaCO2 with pH, bicarbonate, and the clinical context, as described in ABG interpretation.
Sources
- U.S. Food and Drug Administration. Pulse Oximeter Basics.
- American Thoracic Society. Pulse Oximetry. Patient Education Information Series. 2021.
- Davis MD, et al. AARC Clinical Practice Guideline: Blood Gas Analysis and Hemoximetry: 2013.
- Centers for Disease Control and Prevention. Clinical Guidance for Carbon Monoxide Poisoning Following Disasters and Severe Weather. 2026.
Last updated October 28, 2026.