Clinical and Laboratory Standards Institute document EP33—Use of Delta Checks in the Medical Laboratory provides guidance for developing procedures for delta checking and evaluating the differences between consecutive results for the same patient. Delta check alerts refer to situations in which differences between these consecutive results exceed specified limits. Such changes may indicate changes in patient conditions or specimen problems (eg, specimen misidentification, contaminated specimens, hemolyzed specimens). With the growing use of autoverification, delta checks are increasingly used as one of the tools to identify results that need additional review. This guideline represents a consensus of experts who have reviewed available data on approaches for the use of delta checks. It suggests approaches to establishing delta check limits, selecting measurands for which delta checks are useful, developing rules for comparing them to previous results, investigating specimens with delta check alerts, and evaluating the effectiveness of the laboratory's delta check systems.