Oxygen saturation measurement becomes unreliable below what percentage?

• A. 60%
• B. 70%
• C. 80%
• D. 90%

Answer: (B)

Pulse oximetry estimates the fraction of hemoglobin bound with oxygen by sending light through a perfused site and analyzing pulsatile arterial blood. It is most reliable within the device’s validated range, roughly 70–100% saturation, with typical accuracy around ±2–3% when perfusion is adequate. Below about 70%, readings become unreliable because the pulsatile signal weakens and the signal-to-noise ratio worsens, making the calculation more prone to error. Factors like poor perfusion, cold or digital vasoconstriction, motion, nail polish, or ambient light further degrade accuracy in this lower range. Clinically, this threshold means readings under 70% should be interpreted with caution and ideally confirmed with arterial blood gas or co-oximetry if precise saturation is needed.