We propose several approaches to specify trusted region around any given power flow solution in the voltage space. Given a power flow solution is treated as an operating point. The objective is to certify that admissible deviation of current, voltage and/or power of the operating point lies in the trusted region and preserves voltage stability. The trusted region is either an ellipsoid or a convex region described by linear matrix inequality. Then we exploit the computationally tractable sufficient condition (S-procedure for multiple quadratic constraints) for admissible deviation certification. The results are illustrated through the simplest 2-bus system and the IEEE 14-bus test system.