There were 244,852 adult patients admitted during the study period. Of these, 7,402 were excluded because the patient died during the initial hospital stay, 4,658 were excluded because the patient was readmitted less than 24 hours after the initial discharge, and 96,279 were excluded because they had a length of hospital stay of less than 48 hours. The remaining 136,513 admissions formed our study population (). The mean age of the sample (± standard deviation) was 51 ± 18 years, and 53% of the patients were male. The median (interquartile range [IQR]) length of stay of the initial hospitalization was 6 days (IQR, 3–13 days).
Demographic and Clinical Characteristics of Adult Patients Admitted to University of Maryland Medical Center during the Period 2001–2008, by Culture Result
During the study period, 7,898 (6%) of the patients admitted to the hospital had a clinical culture positive for one of the organisms of interest, and 4,737 (3%) had a positive clinical culture obtained more than 48 hours after admission. Of these, 2,318 (49%) of the patients had cultures positive for MRSA, 1,174 (25%) had cultures positive for VRE, and 1,643 (35%) had cultures positive for C. difficile. Three hundred and eighty-three had cultures that were positive for more than 1 organism obtained 48 hours or more after hospital admission. In addition, trend analysis suggested a significant increasing trend in the proportion of clinical cultures obtained 48 hours or more after hospital admission that were positive for MRSA during the period 2001–2008 (P = .02) as well as slight decreasing trends for VRE-positive (P = .06) and C. difficile–positive cultures (P = .05).
Patients with hospitalizations during which a positive clinical culture was obtained more than 48 hours after admission differed significantly from other patients in a number of respects. Patients with a positive clinical culture were significantly older, were more likely to be male and to have an ICU stay, and had a higher mean Charlson comorbidity index. Furthermore, these patients had significantly longer median length of stay (P < .01 for all).
Thirty-five percent of patients were readmitted to the index hospital within 365 days. An examination of survival curves of time to readmission, stratified by clinical culture status, suggests a shorter time to readmission among those patients with a positive clinical culture obtained more than 48 hours after admission (log-rank χ2 value of <.0001; ) Median time to readmission among those with a positive clinical culture was 27 days (95% confidence interval [CI], 25–30 days), compared with 59 days (95% CI, 57–60 days) for patients without a positive clinical culture result. A 30-day period to readmission was also examined. The incidence of 30-day readmission among patients with a positive clinical culture result was 25%, compared with 15% for patients with a negative result or no clinical culture (P < .01). Our final Cox proportional hazards model contained terms for positive clinical culture, sex, age greater than 65 years, length of stay greater than 3 days, Charlson comorbidity index, ICU admission, and year of hospital admission. The results of this model are displayed in . Patients with a positive clinical culture obtained 48 hours or more after admission during their initial hospital stay had a significantly increased hazard of hospital readmission (hazard ratio [HR], 1.40; 95% CI, 1.38–1.51). Additional analysis suggested a greater hazard of readmission for patients with a clinical culture positive for VRE (HR, 1.67; 95% CI, 1.53–1.81), compared with patients with cultures positive for MRSA (HR, 1.30; 95% CI, 1.22–1.38) or C. difficile (HR, 1.35; 95% CI, 1.26–1.45). Furthermore, the inclusion of all 3 organisms in the model resulted in slightly reduced but significant hazards of readmission, which suggests that the presence of a clinical culture positive for each organism is an independent predictor of hospital readmission. Patients aged less than 65 years (HR, 1.45; 95% CI, 1.41–1.49) or who had a length of stay greater than 3 days (HR, 1.14; 95% CI, 1.12–1.17) also had a significantly increased hazard of hospital readmission. Patients with an ICU stay had a decreased hazard of readmission (HR, 0.87; 95% CI, 0.85–0.89). Sex was not a significant predictor of hospital readmission. Additional analyses were performed to determine whether the increased hazard of readmission among those aged less than 65 years was attributable to the inclusion of patients admitted to obstetric or pediatric services (n = 5,886). However, excluding these patients resulted in no change to the HR point estimates. Each unit increase in Charlson comorbidity index score was associated with an increased hazard of readmission (HR, 1.11; 95% CI, 1.10–1.11), whereas each unit increase in year of admission was associated with a decrease in the hazard of readmission (HR, 0.93; 95% CI, 0.93–0.94).
FIGURE 1 Survival curves of time to hospital readmission within 1 year after hospital discharge for those with a clinical culture positive for methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, or Clostridium difficile occurring more (more ...)
Adjusted Hazard Ratio of Readmission within 1 Year after Hospital Discharge for Patients with a Clinical Culture Positive for MRSA, VRE, or Clostridium difficile Occurring More than 48 Hours after Hospitalization