Table 1: Median duration of device use for all patients, those that develop infection and up to infection. Median duration of device use (interquartile range) for all patients on device for all patients that developed the device‐ associated infection up to the first device‐associated infection ventilation 6 (9) 14 (15) 6 (5) central venous catheterization 5 (5) 21 (16) 9 (13) urinary catheterization 6 (7) 17 (17.5) 8 (9) Duration of device use as a risk factor for infection Figure 1 shows the incidence densities of patients at risk that developed an infection, according to the duration of mechanical ventilation, central vascular catheterization or urinary catheterization. The incidence density of CR‐BSI and CA‐UTI varied relatively little according to duration of CVC and CAD use, but that of VAP decreased when the ventilation lasted longer than 9 days. We also calculated the VAP risk per day. Figure 2 presents the risk of patients expressed as a proportion of those at risk for at least the number of indicated days. The risk increased until day 5, remained more or less constant unto day 10 and decreased thereafter. There were too few patients at risk for more than 3 weeks to draw conclusions. Therefore we summarize these. Cox regression takes into account the effect of time at risk, but its relative risks do not give insight into its effect. Logistic regression does not integrate the time at risk in the calculation of its odds ratios. However, this makes it possible to express the effects of discerned periods at risk. Therefore Table 2 shows the odds ratios of increased device use (until infection), determined by univariate logistic regression. Prolonged device use significantly increased the risk of acquiring a device‐associated infection. The risk of CR‐BSI was affected most: the odds ratio for a CVC in situ for 5‐9 days was 4.3 and for a period of 10 days or longer 8.4. Device use affected the risk of VAP the least. Being on a ventilator for at least 10 days was not associated with a higher risk than being ventilated for 5‐9 days. This is reflected in the decreasing incidence density in Figure 1. Other risk factors for infection Incidence densities for different categories of patients are given in the ESM (Appendix B). Table 3 presents the relative risks determined by multivariate Cox regression (univariate results in the ESM, Appendix C). Female sex and SDD use were associated with lower VAP risk. An APACHE II score of 20 or greater was associated with a higher risk. Only SDD use affected the VAP risk significantly. The only independent risk factor for CR‐BSI was acute admission. Acutely admitted patients had a lower risk for CR‐BSI. Independent risk factors for CA‐UTI were female sex, impaired immunity, acute admission, and systemic 26 Chapter 2
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