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We read with interest the recent study on central venous saturation (ScvO2) and mixed venous saturation (SvO2) in 53 patients with severe sepsis [1]. For several reasons, however, we feel the authors' broadly stated conclusion that 'a positive (ScvO2 - SvO2) value is not associated with improved outcome' is unwarranted.  First, in Figure 3 of their paper, graphing the number of measurements rather than the number of subjects is counterintuitive; your readers may find the latter a more clinically useful metric. The meaning of the columns is obscure because a single subject can contribute measurements to both columns (less than and greater than zero).  Second, the total number of measurements depicted in each Figure 3 column is crucial information, but is not provided. After physically measuring the height of the columns, we estimate 278 total measurements, rather than 265 as noted in the text.  Third, we question how the P-value (P = 0.13) for a total population can be numerically equal to the lowest P-value of two constituent groups of similar size.  Finally, the authors should consider acknowledging the possibility of a type II error when stating their conclusion. Specifically, if the total number of measurements were doubled, keeping the proportions the same, the P-value would decrease to 0.08 (Chi square with Pearson's correlation). If tripled, then the P-value becomes P < 0.0001.  We welcome new studies on survival and (ScvO2 - SvO2) difference, and do not believe the current study casts doubt on our finding [2] that a positive difference is strongly associated with survival.

ICU survival and the central to mixed venous oxygen difference