  We apply the flow equation method for studying the current-current response
function of electron systems with the pairing instability. To illustrate the
specific scheme in which the flow equation procedure determines the
two-particle Green's functions we reproduce the standard response kernel of the
BCS superconductor. We next generalize this non-perturbative treatment
considering the pairing field fluctuations. Our study indicates that the
residual diamagnetic behavior detected above the transition temperature in the
cuprate superconductors can originate from the noncondensed preformed pairs.
