The invention relates to methods for indirectly determining clinical laboratory reference intervals. In one aspect, a reference interval is determined using all measurements for a given analyte stored in a large existing database. In other aspects, a characteristic of a subject is used to select a reference population for inclusion in reference interval calculations. In other aspects, the invention provides methods for changing treatment plan, diagnosis, or prognosis for an individual subject based on differences between the new reference interval and a previously utilized reference interval. In other aspects, the invention provides systems and computer readable media for indirectly determining reference intervals.