The integral equation formulation of the Measured Equation of Invariance (IE-MEI) can be considered as a special case of combined field integral equation (CFIE) discretized by the method of moments (MoM), in which the choice of different testing functions for the electric and magnetic fields results in an approximately sparse linear system to solve for the induced current, where most of the matrix elements can be neglected. These new testing functions are numerically derived by a procedure borrowed from the Measured Equation of Invariance (MEI) method, originally developed to find numerically the truncation boundary coefficients of finite difference and finite element meshes. A significant feature of numerically derived testing functions is that they are 'adaptive', i.e., specific to the particular shape of the scatterer boundary and to the location of the function in the boundary.