Does comorbidity affect the outcome of surgery! Total hip replacement in the UK and Japan

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 (23.4%) comorbidity (P < 0.001). In contrast, no significant association was found in Japan. The relationship between comorbidity and change in health status was weak in the UK and non-significant in Japan. Logistic regression confirmed that comorbidity was a significant predictor of serious complications in the UK. The only other significant factor was surgical approach (anterior/antero-lateral; odds ratio 2.16, P < 0.05). Attempts to improve the predictive power of the ICED by modifying its structure and by reclassifying complications was successful in achieving a linear (rather than dichotomous) relationship. The predictive power, however, was poor. Conclusions. 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.