Clinical and Laboratory Standards Institute guideline EP33—Use of Delta Checks in the Medical Laboratory provides guidance for developing a program for a delta check quality control tool to evaluate the differences between consecutive results for the same patient. The delta check program alerts laboratory personnel to situations in which differences between these consecutive results exceed specified limits. Such changes may indicate changes in patient conditions or sample problems (eg, misidentification, contamination, hemolysis). 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 a patient test result to previous results, investigating samples with delta check alerts, and evaluating the effectiveness of the laboratory’s delta check program.

This guideline was revised in 2023 under the Limited Revision Process and replaces the first edition of the guideline, which was published in 2016. Several changes were made in this edition, including: 

• Emphasizing validation of the methods and published results for estimates of biological variation, which are important in setting limits for EP33 

• Aligning this guideline with recommendations of the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM), which uses a strict methodology to assess the validity of published biological variation estimates: 

– For most measurands mentioned in EP33, EFLM has endorsed updated biological variation estimates for both within-subject biological variation (CVI) and between-subject biological variation (CVG). Thus, the calculated indices of individuality shown, which are the ratios of CVI divided by CVG, have been modified. 

– EFLM does not list valid current estimates of CVI and/or CVG for mean corpuscular hemoglobin, partial thromboplastin time, cholesterol (total), or globulins (total). Thus, these measurands were deleted from Table 2. 

• Aligning terminology throughout the guideline

This guideline provides recommendations for evaluating the changes between consecutive test results for the same patient in the same matrix. These evaluations are called delta checks. This guideline reviews the selection and use of delta checks and provides basic information for laboratories that intend to use delta checks. This document considers several uses, including detection of misidentified samples, contaminated or otherwise compromised samples, and clinically significant changes in patients’ test results. This guideline reviews approaches to setting limits for expected differences in consecutive test results, selection of appropriate measurands for use in delta checks, and the types of comparisons that could be used; an approach to evaluating samples that have delta check alerts; and suggested approaches to evaluate the effectiveness of delta checks once they have been implemented. It also provides guidance for defining appropriate follow-up steps for delta check alerts and for the evaluation of the performance of a laboratory’s delta check program. 

The intended users of this guideline are medical laboratory management and personnel. This information may also be of interest to hospital or laboratory informatics staff, and software and medical device vendors who need to understand the laboratory’s goals when implementing an automated delta check program. 

This guideline does not directly discuss informatics aspects (computer programming) for establishing delta checks, or methods for determining the precision of the test methods used.