Although we are in the era of evidence-based medicine, there is still a substantial gap between theory and current practice with the application of reference intervals as decision making tools. Different laboratories may have different reference intervals for the same tests using the same analytical methods and platforms. These differences have the potential to confuse physicians making the assessment and monitoring of patients more difficult by providing discordant information. This paper attempts to demonstrate how to use evidence-based approach for harmonising reference intervals. In order to consider harmonisation we must first have an appreciation of the various factors that influence the determination of that reference interval such as the choice of individuals within the population studied, biological variability of the analyte studied, partitioning, sample collection, analytical aspects such as bias and statistical models.An a priori approach for determining reference intervals, whilst recommended, may be beyond the scope of most laboratories and consideration should be given to the use of a validated indirect a posteriori approach. Regardless of method used, the continuing application of an evidence-based approach in harmonised reference intervals to meet the quality expectations of physicians should be pursued.