There are currently few meaningful data and analyses about variation in the use of pathology tests controlling for patient casemix. Diagnosis-Related Group (DRG) codes are allocated to all patients admitted into a hospital as inpatients. The aim of this study was to use these codes to examine pathology test volumes and variation between hospitals based on linked administrative datasets collected from the pathology service and hospitals. A total of 11,370,000 test orders over six years for inpatients at four hospitals with a single pathology provider were analysed in this retrospective cohort study. The crude and adjusted rates of tests ordered per patient day were estimated using Poisson models. The adjusted rate across all the hospitals revealed a general increase of pathology requests across time from 2008 to 2012. Quality improvements in pathology requesting are dependent on the availability of quality data and meaningful analyses to identify variation between locations across time accounting for differences in patient characteristics. Using DRGs as a way to control for casemix can facilitate the meaningful examination of test utilisation and variation between hospitals and across time to improve quality use of pathology.