A new approach based on the use of interpolation for the electric potential is proposed to increase the accuracy of field calculation using the impedance method. In each voxel of a three-dimensional grid, the electric field is calculated from the potential interpolation function and is used as a solution of the continuity equation. The system of node equations is then solved and the resulting potential distribution obtained allows making the calculation of the electric field and current in each voxel. In this work, the proposed method was used to estimate the sensitivity of a tetrapolar probe in detecting small regions of increased conductivity in a stratified model of human skin. The simulations indicated that inserting a tumor in the epidermis with area 1 mm2 and admittivity fourfold than the normal tissue the load impedance to the current source varies from about 1% while the transfer impedance shows much greater variation, about 8%. Additionally, it was found that the sensitivity of the transfer impedance shows a maximum for electrodes of 1 mm in diameter with spacing of 1.8 mm and for both impedances the sensitivity is independent of the frequency.