Usefulness of biological variation in the establishment of delta check limits
Introduction
Rapid reporting of accurate test results is the main goal of clinical laboratories. In many clinical laboratories, test results are auto verified in order to reduce errors and to report the results more rapidly [1]. Autoverification is a method of examining test results using several tools including quality control results, instrument flagging, moving averages, critical limits, and delta checks [2].
A delta check compares a patient's current and previous results using difference percentage, absolute difference or ratio [3], [4]. A large difference between two consecutive test results is likely indicative of test errors and the patient needs to be retested to rule out errors. Although the delta check is a powerful tool for detecting errors in clinical laboratories, adequate delta check limits are needed. If the delta check limit is set too low compared with biological variation and analytical variation, many test results will exceed the delta check limit. Conversely, if the delta check limit is set too high, many false negatives will result. Therefore, it is crucial to set adequate delta check limits.
Delta check limits have been established using pathologists' experience, reference change values (RCVs) or population distribution of two consecutive test results [5], [6]. An RCV is determined from intra-individual biological variation and analytical variation, and has been considered to be an objective guide for interpreting numerical test results in clinical laboratory serial testing [7], [8]. However, few studies on delta check limits using RCV have been reported. In this study, we examined the difference between the delta check limits by RCV and by the population distribution. We then evaluated the correlation between the intra-individual biological coefficients of variation (CVI) and the delta check limits from the population distribution.
Section snippets
Data collection
The test results for nine routine chemistry tests were collected from the The Catholic University of Korea, St. Vincent's Hospital between January and December 2014. The tests included glucose, aspartate transaminase (AST), alanine amimotransferase (ALT), creatinine, total protein, albumin, sodium (Na), potassium (K), and chloride (Cl). A total of 1,893,955 tests were performed. One year was used for the time interval for the delta checks. These tests were performed using a model 7600–110
Results
A total of 1,893,955 test results were obtained (Table 1). Of all the patients with test results, about 81% (1,533,359) had been previously tested within 1 y. For chloride total test results, 76.3% of patients and for creatinine total test results, 82.8% of patients had previous test results.
The smallest delta check limit of RCV95% was sodium (± 3.1%) and the largest delta check limit was ALT (± 67.6%) (Table 2). Delta check limits of RCV99.9% of the routine chemistry tests were about twice those
Discussion
Our study shows that many test results exceeded the delta check limits by RCV95%, RCV99%, and RCV99.9%. The mean percentage of test results of each item exceeding the delta check limits of RCV95% ranged from 12.3% to 40.6%, which was greater than the expected percentage. In addition, a large difference was observed between the nine routine chemistry tests in the percentage of test results exceeding the delta check limits according to RCV. These findings suggest that the RCV has limitations for
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