In Response

Abstract

We have adapted Bland and Altman’s analysis only in that we recognize that one of our measurements is from a technology widely used in clinical practice.1,2 In our experience, readers are more interested in reviewing differences with respect to a well-established device and that averaging observations from what are essentially a reference and a device under test adds nothing to their understanding. We take this opportunity to clarify that Bland and Altman never use the phrase “confidence interval” in the article referenced by Dr Drummond. They do, however, provide a figure remarkably similar to our Figure 2. We are disappointed that, again, Dr Drummond failed to mention that we qualified and expanded on this analysis by presenting data on subject demographics (Table 1), medical conditions (Table 2), bootstrapped delta with respect to the reference (including 95% limits of agreement; Table 3), bootstrapped concordance and correlation coefficients (Table 4) and statistics on the nonstationary nature of the collected data (Table 4). In all of these cases, the reported statistics were presented separately for volunteers and patients, and then in combination. We can only assume this detail was inconvenient to the message he wished to convey. For at least the past 10 years, it has been well reported that limits of agreement may vary with respect to the sample used in their calculation.3 Like Bland and Altman, earlier authors note that “the limits by themselves provide only a reference interval and should never be used as the determining factor to conclude agreement between two devices.”3 Leaving aside the fact that our analysis was part of a wider statistical treatment, we use the limits precisely as the reference interval for which it is intended. No conclusion of agreement was drawn from this interval. We note with interest that the authors of the articles Dr Drummond cites for describing the additional statistical analysis required4 and for highlighting the underreporting of this analysis5 contain a common member, himself. While we admire this personal campaign, we must point out that providing the confidence interval of the limits of agreement of the statistic in question is beneficial primarily if we want to quantify confidence in conclusions that are being drawn from that data. If we are presenting the limits of agreement as a simple reference interval, then there is a point of diminishing return in providing this statistic of a statistic of some statistics. We disagree that the limits of agreement should answer the question “are these measurement systems equivalent, can I use them interchangeably?” We do not believe that any single metric can answer that question (with or without confidence intervals). We do not, however, see any conflict between his assertion that there is insufficient information in our article to declare whether the systems can be used interchangeably and our conclusions that the technology “may be a useful adjunct to continuous pulse oximetry monitoring” and that that its use “warrants assessment.” Sergio D. Bergese, MDDepartment of AnesthesiologyThe Ohio State University Wexner Medical CenterColumbus, Ohio[email protected] Michael L. Mestek, PhDScott D. Kelley, MDRespiratory and Monitoring Solutions, MedtronicBoulder, Colorado Robert McIntyre Jr, MDDepartment of SurgeryUniversity of Colorado HospitalAurora, Colorado Alberto A. Uribe, MDDepartment of AnesthesiologyThe Ohio State University Wexner Medical CenterColumbus, Ohio Rakesh Sethi, BS, BERespiratory and Monitoring Solutions, MedtronicBoulder, Colorado James N. Watson, PhDPaul S. Addison, MEng, PhDRespiratory and Monitoring Solutions, MedtronicEdinburgh, United Kingdom