Drug Information Journal - March 2009 - (Page 141) Local Laboratory Reference Intervals MEDICAL INFORMATION 141 3 2.5 Harmonization Factor 2 1.5 1 0.5 0 FIGURE 1 Local lab reference intervals for alkaline phosphatase. Relation between the harmonization factor and URL for alkaline phosphatase activity. 0 50 100 150 200 250 300 350 Local Laboratory URL higher URLs. While this example underscores the importance of the calibration process to correct for method differences, which applies to enzymes in particular, it also indicates that there does not seem to be a clear rationale for differences in reference intervals when methods are similar (and hence harmonization factors are similar). In the remainder of this article, this hypothesis is investigated by introducing population characteristics as a further potential criterion for differences in local laboratory reference intervals. varying with time and laboratory. The central laboratory (harmonized) result was used for the subsequent data analysis. This is naturally compatible with the central laboratory reference interval. The local reference interval is before calibration: in consequence, the interval was multiplied by the harmonization factor to obtain an after-calibration reference interval. The harmonization factor for each local laboratory was obtained by comparing the harmonized and local laboratory results as described above. ALERT FLAGGING VERSUS LOCAL AND CENTRAL LABORATORY LIMITS Due to differences between local reference intervals and the central laboratory reference interval, results will be flagged differently. Here we examine leucocyte count and use the following terminology: • Lab ID: This is the numerical local laboratory code by which the laboratory is identified in the data set: Table 2 reports only the 20 laboratories with the highest number of samples (182 or more). • N of results: This is the number of valid results for each local laboratory. A result is valid if both the local and the harmonized or pooled values are recorded in the data set, accompanied by both local and central reference intervals. There were M AT E R I A L S A N D M E T H O D S HYPOTHESIS Eight analytes were selected for demonstration of the method, as shown in Table 1. Four were hematology analytes and four were clinical biochemistry analytes. The total number of observations was 94,146. This is the total number of test results across patients, not stratified according to age or gender and distributed over the analytes as shown in Table 1. The hematology analytes were not calibrated: the central laboratory result is the same as the local laboratory result. The clinical biochemistry analytes were calibrated: the central laboratory result is a multiple of the local laboratory result, resulting in a harmonization factor, Drug Information Journal
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