Nephrology and Dialysis Unit, Hospital of Martina Franca, Italy
Sir,
We read with interest the paper Variability of relative blood volume during haemodialysis by Krepel et al. [1]. They conclude that relative blood volume (RBV) monitoring is of limited value in the prevention of dialysis-related hypotension due to the large intra- and inter-individual variability [1].
We were struck, when analysing Tables 3 and 4 of the paper by Krepel et al. [1], by the data of patient 4, which are considerably different from those of the other nine patients of the study. We were confirmed in our opinion that patient 4 must be considered an outlier, from the statistical point of view, and by that way excluded from the statistical analysis, when examining the individual behaviours of RBV changes plotted against time and ultrafiltration volume (UV) (Figures 2a and 2b): in these graphs there is one single patient whose percent RBV increases consistently during ultrafiltration, at least in the first 60 min of dialysis: this is really a very bizarre behaviour! We hypothesize that this patient corresponds to patient 4 of Tables 3 and 4.
To consider patient 4 an outlier is a crucial point in the interpretation of the results of the present work, due to the small number of patients (only 10) enrolled into the study.
Thus, if we reanalyse the data shown in Table 3 excluding patient 4, we can see that both intra- and inter-individual variability decrease considerably (Table 1): the mean intra-individual coefficient of variance (CV) by a percentage ranging from 18 (at 240 min) to 54% (at 120 min); the inter- individual CV by a percentage ranging from 15 (at 60 min) to 34% (at 180 min). As shown in Table 1
, the largest differences are observed in mean intra-individual CV.
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In conclusion, both intra- and inter-individual variability do exist, but they are not as large as suggested by Krepel et al. [1].
References