The present application relates to continuous measurement of total hemoglobin (tHb) in whole blood. In one embodiment, different wavelengths are used for normalization of the spectral intensity and calculation of the total hemoglobin. In particular, for normalization, a first wavelength is used wherein the wavelength is substantially insensitive to changes in levels of hemoglobin and oxygen saturation. For calculation of the total hemoglobin, a second wavelength is used. The second wavelength is sensitive to changes in levels of hemoglobin, but substantially insensitive to changes in levels of oxygen saturation. In another embodiment, a continuous measurement can be made using two wavelengths that are both sensitive to oxygen saturation, but they both are equally sensitive. In other words, the normalized intensities associated with the two wavelengths change equal amounts with equal changes in oxygen saturation levels.
