The value of risk scores for predicting postoperative nausea and vomiting when used to compare patient groups in a randomised controlled trial

Whilst conducting a randomised controlled trial into the effects of combination anti-emetics, we endeavoured to confirm that our patient groups were matched using the predictive scoring systems for postoperative nausea and vomiting (PONV) and postoperative vomiting (POV) reported in the literature. One hundred and seventy-seven female patients attending for day case gynaecological surgery were studied and their individual risks of PONV and POV were calculated using four predictive models for PONV and two predictive models for POV. The scoring systems were then evaluated to see if agreement existed between them using the method described by Bland and Altman. Bias and 95% limits of agreement were calculated for each combination. Agreement between scoring systems was poor. As the scoring systems gave widely divergent predictions, we concluded that the predictive risk for PONV or POV would be dependent upon the scoring system chosen, thus limiting their usefulness in this role.