Numerical experiments were performed to evaluate the applicability of nonperturbing techniques for inferring the reactivity of fast reactors. Almost all such techniques contain the tacit assumption that the flux throughout the system is described by the fundamental mode and that higher modal contamination is negligible. Results were obtained for four reactive states of a typical 1000-liter-core Liquid Metal Fast Breeder Reactor (critical, -1$, -7$, and -30$) which illustrate the range of applicability of the point-model assumption with first-order corrections.