Interleukin 17 and 10 in relapsing remitting multiple sclerosis

With great interest we read the study by Tawfik et al. [ ] entitled, “Interleukin 17 and 10 in a sample of Egyptian relapsing remitting multiple sclerosis patients”. In this study, the authors investigated serum levels of IL-10 and IL-17 in patients with relapsing remitting (RR) MS during relapse and remission phases and controls. Although it was mostly a well-designed study, we have the following comment from a statistical point of view. In the statistical analysis, the authors stated that they used Mann-Whitney test for not normally distributed quantitative variables. In this study, comparison of IL-10 and IL-7 in two phases of MS and in healthy controls was performed using Mann-Whitney test. Mann-Whitney test is used to determine whether there are any statistically significant differences between the means of two independent (unrelated) groups. But the groups of this study were not independent, because they did not use two different groups of MS patients as relapsing and remitting groups. Indeed, they used 30 RRMS patients as both relapsing and remitting groups in two different times based on the phases of disease. Therefore, two groups of this study are dependent and using the Mann-Whitney test is not appropriate. After assessment of the normal distribution of quantitative variables, they must use dependent t-test or Wilcoxon for comparison of the mean levels of IL-10 and IL-7 between relapsing and remitting phases of the disease.