The major role of plasmonics in the development of highly sensitive and selective biosensor is a known fact. However, the nanoscale metallic sensitive part of biosensors presents some specific problems. Indeed the measurement of optical properties of nanostructured materials (depending on shape, size, annealing) gives results with large uncertainties. Nevertheless the models of biosensors are rarely used to propagate these uncertainties and to determine numerically their influence on the efficiency of such a biosensor. To fill this gap a Monte-Carlo method is proposed to characterize the influence of uncertainties on the optical properties of gold on the performance of Surface Plasmon Resonance (SPR) biosensor. The uncertainty on the optical properties of gold is shown to be more critical for the field enhancement that probes the biological molecules than for the reflectivity.