What is SpO2 in Photoplethysmography?
SpO2 (peripheral oxygen saturation) measures the percentage of hemoglobin in arterial blood saturated with oxygen. PPG-based pulse oximeters estimate SpO2 by comparing light absorption at red (~660 nm) and infrared (~940 nm) wavelengths, exploiting the different absorption spectra of oxygenated and deoxygenated hemoglobin.
Photoplethysmography-based SpO2 estimation relies on the Beer-Lambert law. Oxygenated hemoglobin (HbO2) absorbs more infrared light, while deoxygenated hemoglobin (Hb) absorbs more red light. A two-wavelength sensor computes the ratio R = (AC_red/DC_red) / (AC_IR/DC_IR) and maps it to SpO2 via an empirical calibration curve derived from controlled human studies.
Consumer wearables typically achieve ±2–3% accuracy in the 95–100% range under resting conditions. Accuracy degrades significantly during motion, poor perfusion, high ambient light, and at low SpO2 values (<90%). FDA clearance for clinical SpO2 monitoring requires ±3% accuracy across the 70–100% range in diverse skin tones. Recent studies highlight that melanin in darker skin tones can cause systematic overestimation of SpO2 by up to 4%, a bias now under active regulatory scrutiny.
Clinical-grade pulse oximeters use probe designs that minimize ambient light interference and apply sophisticated motion artifact rejection algorithms. The TROIKA and JOSS frameworks introduced adaptive filtering with accelerometer reference signals to recover SpO2 estimates during physical activity, achieving RMS errors below 2% in running scenarios.
Frequently Asked Questions
How accurate is wearable SpO2 compared to clinical pulse oximeters?
Consumer wearables typically achieve ±2–3% accuracy at rest, versus ±1–2% for FDA-cleared clinical devices. During motion, wearable accuracy can degrade to ±5% or worse.
Why does SpO2 accuracy differ by skin tone?
Melanin absorbs light at wavelengths used for SpO2 measurement, reducing the signal-to-noise ratio and causing systematic overestimation of SpO2 in darker skin tones by 1–4% in some devices.
Can PPG measure SpO2 with a single wavelength?
No. Conventional SpO2 requires at least two wavelengths (red and infrared). Single-wavelength PPG can only estimate relative perfusion, not absolute oxygen saturation.