To compare nosocomial infection rates estimated in different time periods or in different hospitals, it is necessary to control for differences in the distribution of factors that substantially influence a patient's susceptibility to infection. To evaluate the associations of multiple risk factors with the occurrence of infection at each of four major sites and to develop composite measures for use in controlling for differences in the distribution of risk among groups of patients, we used a multivariate categorical data analysis technique to study the infection experience of 169,518 patients admitted in 1970 to the 338 hospitals studied in the Study on the Efficacy of Nosocomial Infection Control (SENIC Project). The relative importance of risk factors and their complex interactions varied by site. The factors found to be highly important for one or more sites were duration of urinary catheterization, the patients' intrinsic risk as reflected in their diagnoses and types of surgical procedures, duration of preoperative hospitalization, duration of operation, anatomic location of surgical procedure, previous infection and steroid or immunosuppressive therapy. Site-specific risk strata and estimates of each patient's probability of acquiring infection were developed from these data for use in future SENIC analyses.
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