A method 100 and apparatus for determining SpO2, of a subject from an optical measurement is disclosed herein. In a described embodiment, the method 100 includes obtaining a PPG(red) signal and a PPG(IR) signal at steps 102 and 104, and pairing the PPG(red) and PPG(IR) signals at step 106 in which an amplitude of each cardiac rhythm cycle of the first signal is aligned to an amplitude of a respective cardiac rhythm cycle of the second signal to form a plurality of paired windows. At step 108, the method further includes, for each paired window, calculating values of a ratio R from the paired first and second signals and based on the calculated R values, binning the calculated R values into predetermined frequency bins at step 110. At steps 112 and 1 14, at least one of the frequency bins is selected to derive a revised ratio, Rrev and at step 1 16, Sp02 is derived from the revised ratio Rrev. A zoning schema for deriving SpO2 is also disclosed.
