Systems and methods for estimating a saturation level of oxygen in hemoglobin (SpO2) are provided. In some aspects, a system includes a detector module configured to receive an oximeter output signal indicative of light absorption in a patient. The oximeter output signal alternates between infrared light components and red light components, and includes a first portion obtained at least partly during switching from at least one of the infrared components to at least one of the red components. The oximeter output signal also includes a second portion obtained at least partly during switching from at least one of the red components to at least one of the infrared components. The system also includes a processing module configured to estimate an SpO2 of the patient as a ratio between (i) a time derivative of the first portion and (ii) a time derivative of the second portion.
