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[The usefulness of intrinsic risk factors of infection of the surgical site as predictors of other infections and death].

BACKGROUND
The use of two indices of patient intrinsic risk of surgical site infection, the SENIC index and the NNIS index, have been recommended to adjust for the case-mix and improve the comparability of infection rates across different hospitals. We assess the usefulness of the indices in predicting other infection sites and in-hospital death.


METHODS
A prospective study on 1,483 patients admitted to a service of general surgery was performed. The outcomes were nosocomial infection and in-hospital death. Relative risk and its 95% confidence interval were estimated. Unconditional logistic regression analysis was used to adjust for potential confounders.


RESULTS
In the follow-up, 155 surgical site infections (10.5%), 19 postoperative pneumonias (1.3%), 33 urinary tract infections (2.2%) and 33 in-hospital deaths (2.2%). Both the NNIS index and the SENIC index showed a clear trend with infection risk and death; the higher the index, the higher the risk. After controlling for several confounders, the association was still observed. To assess whether the SENIC index added predictive power to the NNIS index (and vice versa) a linear regression analysis of the SENIC index on the NNIS index. A set of residuals (difference between the observed and expected values according to the regression equation) were estimated. In the logistic regression models, residuals of the NNIS index (part of the NNIS index unrelated to the SENIC index, whereas the residuals of the SENIC index did not increase the predictive power of the NNIS index.


CONCLUSIONS
Both the SENIC index and the NNIS index are independent predictors of several sites of nosocomial infection and in-hospital death. The NNIS is a better predictor than the SENIC index.