Abstract
Objectives. To assess the impact of comorbidity on the outcome of surgery in the UK and in Japan; to determine the predictive ability of a new measure of comorbidity, the Index of Co-Existent Diseases (ICED); and to see if its predictive power could be improved. Design. Logistic regression using data from two retrospective cohorts with prospective outcome data collection. Setting. Six hospitals (three teaching, three non-teaching) in the UK and 15 (12 teaching, three non-teaching) hospitals in Japan. Study participants. Patients undergoing total hip replacement (THR) surgery in the UK (n = 268) and in Japan (n = 249). Main outcome measures. Serious complications before hospital discharge and change in three measures of general health status [basic activities of daily living (ADL); instrumental ADL; social activities]. Results. The distribution of levels of comorbidity differed between the UK and Japan: none (26.1 versus 42.2%); mild (30.6 versus 43.0%); moderate (23.5 versus 12.0%); and severe (19.8 versus 2.8%). In the UK, the incidence of serious complications was higher in patients with moderate (27.0%) or severe (26.4%) comorbidity than in those with no (14.3%) or mild (13.4%) comorbidity (PPConclusions. Comorbidity is a significant determinant of serious complications following THR but not of changes in functional or health status. Comparisons of clinical performance using post-operative complications must take levels of comorbidity into account if they are to be meaningful. The ICED is of less validity in the UK and Japan than in its country of origin, the USA. Further work to develop better instruments for the UK and Japan is needed.

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