This paper addresses the problem of hospital stay length as a risk factor for nosocomial infection and as a modifier of the effect of other risk factors for hospital infection. Patients were selected form two cross-sectional studies done in two different seasons of 1986. Risk of infection rose fairly steadily as hospital stay length increased (correlation coefficient: 0.83, p less than 0.01). Several risk factors (operation, underlying disease, and age) were analyzed on the basis of 1) raw data and 2) data stratified by length of stay. The results showed that hospital stay length is a strong modifier of the remaining risk factors, generally reducing, their effect on the development of hospital infection as length of stay increases.

To evaluate the relationship between severity and nosocomial infection in critical patients, we have conducted a prospective study at the Intensive Care Unit of the University of Granada Hospital (Spain). Patients' severity was evaluated by APACHE II and TISS. We found a positive association with nosocomial infection risk for an APACHE II score greater than 12 points (RR = 2.45) and for first-day TISS greater than 20 points (RR = 2.51). With a multivariate analysis we did not find an association between APACHE II and nosocomial infection risk, but each TISS point suggests an infection risk increment of 6%. We concluded TISS may be considered a good infection risk predictor. TISS could also be taken into account when nosocomial infection rates from several ICUs are compared.

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To evaluate the relationship between duration of preoperative stay and the risk of nosocomial infection, we studied 449 patients who underwent surgery at the University Hospital of Granada during the first six months of 1986. Patients were chosen from two cross-sectional surveys. Nosocomial infection was studied throughout each patient's hospital stay. Other variables included preoperative stay, age, severity and total length of stay. The data were analyzed by comparing nosocomial infection for different lengths of preoperative hospitalization, age and severity by calculating the Odds Ratio. The effects of age and severity were studied by stratifying patients by duration of preoperative stay. Two multivariate regression models were used to confirm the results of the stratified analysis. The results suggest that lengthening the preoperative stay may increase the risk of nosocomial infection in surgical wounds and in other sites, and may simultaneously potentiate the effects of other risk factors such as age and severity of the patient's condition, whose influence on susceptibility to infection increases with the duration of preoperative stay.