Gas turbine driven centrifugal compressors often undergo a detailed performance analysis either for the acceptance by the user or to evaluate their existing condition. Often, the tests are conducted at site, where the compressor and its driver must prove their capability to fulfill the requirements of the project specifications. Typical field testing of gas turbine compressor packages requires the evaluation of head, efficiency, capacity, fuel consumption, as well as the available driver power. The process is not straightforward since the conditions at which the compressor package is tested are typically different from the originally agreed upon conditions. In particular, the ambient conditions, as well as the load conditions at the test are usually different. There are a number of standard procedures for carrying out a field test. In this work, one of them is considered for quantifying how the variation in some parameters can impact the result of the field test. Particular attention is given to the evaluation and data correction for the gas turbine. The results of the test can be affected by several factors. Some of them are related to the installation, such as the array of RTDs used. Further, the influence of the accuracy of the input data that are used for the calculations must be considered by determining the effect of test uncertainty. Other parameters that can affect the results are related to the modeling: The natural gas exhibits real gas behavior at the test conditions, and an equation of state has to be used for data conversion. The choice of an equation of state can translate into a differences in the test results. An assessment of the impact of these factors on the outcome of a field test is reported. From the results of this work, the expected error as consequence of deviation from the specification can be quantified.