Operational reliability is normally checked using contingency analysis, thus requiring power flow solutions under a wide variety of system conditions. As power flows are computationally expensive and approximate solutions are usually acceptable, fast estimation methods are used. Real power distribution factor methods provide a good trade-off between accuracy and speed. Reactive power and voltage sensitivities-based methods have not been as successful, due to the large maximum estimation errors reported. This paper discusses the estimation of post-contingency voltages and reactive power generation and flows using sensitivities. Contingencies that lead to large estimation errors are detected using the nodal power mismatches of the estimated state. Representative results are presented using the IEEE 57-bus test system.