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.
Performance varies by device, population, saturation range, placement, and conditions, so a single error range should not be generalized to all consumer wearables. Accuracy often degrades during motion, poor perfusion, high ambient light, and at lower saturation. FDA guidance recommends controlled desaturation testing over 70–100% SaO2 and reporting accuracy as Arms. Its January 2025 draft proposed broader and more objective evaluation across skin pigmentation, but those proposals are non-binding until finalized. Clinical studies have found higher rates of occult hypoxemia in some patients identified as Black, although the error cannot be reduced to one universal skin-tone correction.
Clinical-grade pulse oximeters use probe designs that minimize ambient light interference and may apply motion-tolerant signal processing. TROIKA and JOSS are research frameworks for estimating heart rate from motion-corrupted wrist PPG; they do not estimate SpO2 and should not be used as evidence for oxygen-saturation accuracy.
Frequently Asked Questions
How accurate is wearable SpO2 compared to clinical pulse oximeters?
Accuracy is device- and condition-specific. Compare validation against arterial co-oximetry using Arms and bias across the tested saturation range; resting agreement near normal saturation does not establish performance during motion, poor perfusion, or hypoxemia.
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.