Systems and methods for processing photoplethysmograph (PPG) signals to determine a differential pulse transit time (DPTT) are disclosed. Sensors may be used to obtain first and second PPG signals from a subject. The sensors may be placed at different locations on the subjects body. A first algorithm may be performed on the PPG signals or on signals derived from them to obtain a DPTT. A corresponding confidence measure may be determined and if the confidence measure falls within a first numerical range, the calculated DPTT may be used. On the other hand, if the confidence measure falls within a second numerical range, an alternative algorithm may be performed on the PPG signals or on signals derived from them and the DPTT obtained using the alternative algorithm may be used. The DPTT may be used to perform continuous or periodic measurements of blood pressure.
