Hemodialysis access flow measurements using flow dilution and in-line dialysance are not interchangeable for hemodialysis access flow surveillance.

To the Editor: In their recent article, Whittier et al.1 present a 48-patient study comparing two methods for surveillance of hemodialysis access flow velocity dilution (FD) (Transonic HD02, Transonic Systems, Inc., Ithaca, NY) and in-line dialysance (DD) (Fresenius Medical Care North America, Waltham, MA). Unfortunately, the authors made an error in the statistical assessment of the results. After correction of the statistical error, the conclusion of the article is unfounded: the study suggests instead that measurements of access flow by FD and DD are not interchangeable. The Bland-Altman analysis evaluates the closeness of two methods through calculation of the difference between each measurement pair and can determine the presence of systemic overestimation or underestimation through mean (bias) and the magnitude of the difference through calculation of the standard deviation (SD). The Bland-Altman test is usually presented as mean SD, an earlier study2 cited by the authors for comparison (135 229 ml/min) was presented in this fashion, and I assume that the authors present their data as such as well. Both in the Results and in the Discussion section, the authors report that the Bland-Altman analysis indicate that “FD flows were greater than DD values by a mean difference of 29 19 ml/min.” There is an error in the authors’ calculation of SD. The authors’ reported bias of 29 ml/min seems supported by their Figure 2. However SD 19 ml/min is impossible: if this were true, 96% of all differences between FD and DD should lie within the band of 29 2SD 9 to 67 ml/min. It is actually vice versa: the great majority of differences are outside this band. From the above Figure 2 of Whittier et al., one may estimate that approximately two third of the data points fell within 29 180 ml/min, so SD 180 ml/min for this study. This 10 times larger correction is important, because the SD, according to the literature, is considered the main comparison criterion. For example, in the field of blood flow measurements such as cardiac output, the most commonly used approach is Critchley and Critchley,3 which includes a calculation of percent error that equals the ratio of 2SD to the mean. This ratio should be 30% to consider a new cardiac output method interchangeable with the traditional Swan-Ganz cardiac output reference method. It is based on a quite liberal assumption that the reproducibility of a reference method and a new method are 20% for 96% of the data. Considering that for this article, Bland-Altman SD 180 ml/min and mean flow 1,016 ml/min, the 2SD percent error is 36% ( 30%). For shunt flow surveillance, this difference between methods is especially high in view of the K/DOQI shunt flow trending guideline, which seeks to discern a 25% drop during a 4-month period 25%. In the discussion section, Whittier et al. compared their results with a previous analogous study by Lacson et al.2 (There is a mistake in this citation, the authors referenced a Gotch article4 instead of Lacson et al.). Whittier et al. comment on the increased agreement between the two methods: mean difference 29 19 ml/min in their study versus 135 229 ml/min in Lacson et al. They suggest that this may stem from “improved design and calibration of newer models used in the current study. ” After correction for SD 180 ml/min in this study, the difference with SD 229 ml/min in Lacson et al. is no longer significant, especially considering that the access flow range for comparison was cut in this study to 2,000 ml/min versus 2,500 ml/min in Lacson et al. If we consider only data in the same 2,000 ml/min range in Lacson et al.2 it yields SD 190 ml/min, thus demonstrating that the disagreement between methods did not become improved in the new Fresenius equipment. In conclusion, the application of a quite accommodative criterion to compare Transonic access flow dilution technology with Fresenius in-line DD technology suggests that they are statistically not interchangeable.