Accurate and timely well rate tests ensure prompt operating decisions and effective reservoir management, especially for shut-off of any unwanted fluid. A conventional well testing system typically employing a two-phase separator and single-phase flow meters has widely gained industrial acceptance in highly productive oil fields. Because of full range water cut capability, immunity to fluid compositional change and flow disturbances, no wearing parts and high measurement accuracy, a Coriolis flow meter as a single-phase flow meter gained popular installation in such conventional well testing systems over the last few decades. Positive results had been produced from such Coriolis flow meters in one of Saudi Aramco's operating oil fields over the last decade. Consequently, as a result of invasion of gas cap gas, the existing testing facility could not provide very reliable well testing results recently in this oil field. Testing deficiency significantly impacted the reservoir management decisions for effective shut off of excessive gas production in some oil producers. Intensive well rate testing campaigns using different technologies, including multiphase flow meters (MPFMs) were conducted. The analysis of testing results revealed that some factors greatly impact the measurement accuracy from a Coriolis flow meter, which primarily led to the testing deficiency in this field. Discussed and analyzed in this paper is how the quality of the testing data is affected by the influential variables, testing facility configuration, and operating process. The root causes of testing deficiency were identified and specific solutions to solve those issues proposed. Innovative flow calculation formulae are developed to rectify the reported testing data and verified by other source testing data. As a result, testing efficiency was improved by 30%~60% by the existing testing facilities in this field. Also described in this paper are lessons learned, which will enrich the technical knowledge base. As a result of this work, an approximate $3 million investment could be avoided to replace the existing 30 plus testing facilities with new MPFMs. Introduction Well testing always provides critical decision making information at every stage of an oil field's development life. At the early stage during the oil exploration, well testing helps the engineers to confirm whether the underground hydrocarbon reserve can be economically tapped using whatever technology is available in the market and to prove the estimated reserves. During the normal development of a field or a reservoir, well testing provides petroleum engineers with insight into the production potential of the reservoir and with the physical evidence of well conditions. Unexpected changes, such as extraneous water or gas production may signal well or reservoir problems. Abnormal production declines may be caused by artificial lift problems, scale buildup in the perforations, etc. All information provided by well testing enables early detection of any wellbore problems and allows the engineer to take remedial actions quickly, to ensure optimum production from a well, and to have more confidence in production changes as a result of well workovers, wellbore stimulation or treatment. In brief, accurate and timely well testing information always plays a critical role in reserve estimation, production forecasting, production optimization, capital and operational decisions during the entire development life of a reservoir.