An apparatus including an input interface configured to provide signals from at least two sensors for at least two postures including: signals, dependent upon blood presence, from a first sensor when a subject is in a first posture signals, dependent upon blood presence, from the first sensor when the subject is in a second posture signals, dependent upon blood presence, from a second sensor when the subject is in the first posture and signals, dependent upon blood presence, from the second sensor when the subject is in the second posture and processing circuitry configured to determine and output a metric by combining, according to predefined calibration data the provided signals.
