Randomised controlled trials: subgroup analyses

The effectiveness of wound edge protection devices in reducing surgical site infection after abdominal surgery was investigated. A randomised controlled trial was undertaken. The intervention was standard intraoperative care plus use of a wound edge protection device during the intra-abdominal part of the operation. The control was standard intraoperative care alone. The sample size for the trial was based on having 80% power to detect a 50% reduction in the infection rate, assuming a rate of 12% under standard care. A total sample size of 710 participants (355 in each arm) was needed. It was necessary to recruit 750 patients to the trial to allow for an estimated 5% dropout rate. In total, 760 patients undergoing laparotomy were recruited; 382 patients were allocated to the intervention group and 378 to the control group.1

The primary outcome was surgical site infection within 30 days of surgery, as assessed by clinicians who were blind to the patients’ device group allocation. Secondary outcomes included quality of life and duration of hospital stay. Subgroup analyses that compared surgical site infection within 30 days of surgery between the device groups were performed, with the aim of establishing whether the intervention was beneficial in particular groups of patients. The analyses incorporated 15 characteristics of the patients and the operations.

Six patients in the intervention group and five in the control group did not undergo laparotomy. Furthermore, seven patients in each group were lost to follow-up. Although the proportion of patients with surgical site infection was lower in the intervention group, the difference was not significant (24.7% (n=91) v 25.4% (n=93); odds ratio 0.97, 95% confidence interval 0.69 to 1.36; P=0.85). No significant difference was seen between the groups in the secondary outcomes (quality of life and duration of hospital stay). The subgroup analysis did not …