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Letters to the Editor |
Department of Pathology, University of Utah, Health Sciences Center, Salt Lake City, UT
Address for correspondence: ARUP Laboratories, 500 Chipeta Way, Salt Lake City, UT 84108. Fax 801-584-5207; e-mail william.roberts{at}aruplab.com.
To the Editor:
We appreciate the information provided by Waskiewicz et al. in their letter. They are correct that the study by Ognibene et al. (1) refers to a second-generation thyrotropin (TSH) assay on the ADVIA Centaur and not to a third-generation TSH assay. We regret this error. The study by Vogeser et al., cited as Ref. 4 by Waskiewicz et al., did not actually include an estimate of the functional sensitivity, but rather imprecision was 22.3% at a TSH concentration of 0.014 mIU/L and 3.9% at 0.26 mIU/L (2). These are not sufficient data to estimate functional sensitivity.
The major issue is why our study yielded a higher functional sensitivity than theirs did. They indicate that each pool was tested with all reagent lots in one run. Our study used each of two reagent lots sequentially, which might in part account for the higher imprecision (3). The instrument in their study was used for various patient sample evaluations in support of Centaur customers. The instrument in our study was used for routine testing of patient samples in a reference laboratory setting with 
10 000 patient results reported monthly, and TSH-3 was one of the analytes being routinely reported. The differing environments and use of the ADVIA Centaur analyzers in these 2 studies may have contributed to differences in imprecision. We maintain that our experimental conditions are more representative of what will be encountered in routine clinical testing.
It is unclear whether authors of previous studies have performed imprecision studies in a research setting or in a clinical testing environment. To our knowledge, no one has reported on the effects of increasing workload on assay imprecision, but this may be a factor affecting the precision of some analyzers. A better understanding of which variables are most important and how they affect assay imprecision could lead to better assay performance during routine clinical use. In the study by Waskiewicz et al., the functional sensitivity of lots 38 and 41 of TSH-3 reagent was 0.022 mIU/L, whereas that of lots 26 and 29 (the ones used in our study) was 0.012 mIU/L. It would be interesting to field-test lots 38 and 41 to see whether the increased functional sensitivity exhibited by these two lots in a controlled setting would also be evident in routine clinical testing.
References
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