Understanding increased mortality in Clostridium difficile-infected older adults.

We read with great interest the study by Walker et al examining the impact of Clostridium difficile infection (CDI) on 14-day attributable mortality [2]. The authors drew conclusions about C. difficile genotype-specific mortality, noting higher attributable mortality for clades 2 (ribotype 027) and 5 (ribotype 078) [2]. Although the statistical methods appear to be adequate, the presentation of the results adopted by the investigators does not fully support their conclusions. The authors report unadjusted and adjusted P values (Table 1 [2]) for the factor “type of test,” indicating that the 7 levels (EIA [enzyme immunoassay]–positive, culture-negative; EIA-positive, not cultured; clade 1, etc) had significantly different hazard ratios for mortality compared to the reference (EIA-negative) [2]. The authors concluded that mortality was significantly different between clades (worse for clades 2 [ribotype 027] and 5 [ribotype 078]), but the P values pertained to levelwise comparisons to the EIA-negative cases and not between-clade comparisons. Similarly, the P values shown in Walker et al’s Figure 3 represented differences compared to EIA-negative cases (ie, 1.00) and not between clades [2]. Thus, it is unclear whether any significant betweenclade differences exist after adjustment for host factors. We agree that CDI, regardless of C. difficile clade, resulted in increased mortality compared to patients with non-CDI diarrhea. However, determining