Digital holographic microscopy (DHM) is a technique that allows obtaining, from a single recorded hologram, quantitative phase image of living cell with interferometric accuracy. Specifically the optical phase shift induced by the specimen on the transmitted wave front can be regarded as a powerful endogenous contrast agent, depending on both the thickness and the refractive index of the sample. We have recently proposed a new and efficient decoupling procedure allowing to directly obtain separate measurements of the thickness and the integral refractive index of a given living cell. Consequently, it has been possible to accurately measure (with a precision of 0.0003) the mean refractive index and the volume of living erythrocytes. Here, application of this decoupling procedure on erythrocyte allows to measure a refractive index of 1.40 and a mean volume of about 106 μm3.