Multiplicative Surrogate Standard Deviation: A Group Metric for the Glycemic Variability of Individual Hospitalized Patients
Objective: Group metrics are described to quantify blood glucose (BG) variability of hospitalized patients. Methods: The “multiplicative surrogate standard deviation” (MSSD) is the reverse-transformed group mean of the standard deviations (SD) of the logarithmically-transformed blood glucose (BG) data set of each patient. The “geometric group mean” (GGM) is the reverse-transformed group mean of the means of the logarithmically-transformed BG data set of each patient. Before reverse-transformation is performed, the mean of means and mean of SD’s each has its own SD, which becomes a multiplicative standard deviation (MSD) after reverse-transformation. Statistical predictions and comparisons of parametric or nonparametric tests remain valid after reverse-transformation. A subset of a previously-published BG data set of 20 critically ill patients from the first 72 hr of treatment under the SPRINT protocol was transformed logarithmically. After rank-ordering according to the mean of the SD of the logarithmically-transformed BG data of each patient, the cohort was divided into 2 equal groups, those having lower or higher variability. Results: For the entire cohort, the GGM was 106 mg/dL (÷/× 1.07), and MSSD was 1.24 (÷/× 1.07). For the subgroups having lower and higher variability respectively, the GGM in mg/dL did not differ, 104(÷/× 1.07) vs. 109 (÷/× 1.07), but the MSSD differed, 1.17 (÷/× 1.03) vs. 1.31 (÷/× 1.05), p = 0.00004. Conclusion: By using the MSSD with its MSD, groups can be characterized and compared according to glycemic variability of individual patient members.