acinetobacterbaumannii in nursing homes 1157
and Laboratory Standards Institutes criteria and defined breakpoints.38 Tested antibiotic classes included (1) penicillin combinations (ampicillin/sulbactam and piperacillin/tazo- bactam); (2) aztreonam; (3) first-, third-, and fourth- generation cephalosporins; (4) aminoglycosides (gentamicin
and tobramycin); (5) quinolones (ciprofloxacin and levo- floxacin); (6) carbapenems (imipenem and meropenem); (7) tetracycline; and (8) trimethoprim/sulfamethoxazole. Topical and antiseptic agents were not included in antimicrobial profiles. A. baumannii strains resistant to 3 or more classes of antibiotics were considered MDR.29
Statistical Analyses
Data were entered into a master database (Access 2007; Microsoft) and analyzed using Stata, version 13.1 (StataCorp). The associations of key predictors with MDR A. baumannii colonization were examined using exact logistic regression adjusting for NH site to appropriately account for nesting and small sample sizes. Clustered logistic regression was used with exact logistic regression models for 2 reasons: (1) the clustered element was to account for the dependencies in the outcomes by facility, and (2) the exact logistic framework was used to address parameter estimation for small sample sizes. To explore more-complex associations among several predictors and MDR A. baumannii colonization, all prior statistically significant variables that had enough data were included in an augmented logistic model. The multivariable model was empirically driven on the basis of results from the previously run bivariate models using alpha≤.05.
results Study Population
In the 6 control NHs, 215 residents or their legal guardian provided consent and were enrolled in the TIP study. No cases of MDR A. baumannii colonization occurred in 2 facilities, thus controls from these 2 NHs were excluded from further analysis (n=47). The number of cases at the 4 remaining facilities varied, with facility E contributing the most (12 [48% of cases]) and facility G contributing the fewest (1 [4% of cases]). Facilities also differed in the degree of short- vs long- term care; the median length of stay among enrolled residents at facility K was 170 days compared with other facilities with median stays less than 90 days. Of the 168 residents with an indwelling device at 4 facilities, 25
(15%) were colonized at least once with MDR A. baumannii, all of which were resistant to cephalosporins, monobactam (aztreonam), and quinolones. Twenty-four cases (96%) carried A. baumannii resistant to 9 or more commonly used antibiotics. Most cases (20 [80%]) were colonizedwith carbapenem-resistant A. baumannii, accounting for 68.9% of isolates (93/135). Of the 25 cases, 16 (64%) were preexisting, or present on study enroll- ment, whereas 9 (36%) were new acquisitions. Eleven cases
Risk Factors for MDR A. baumannii Colonization
Clustered exact logistic regression models showed that colo- nization with MDR A. baumannii was significantly associated with functional disability. A high level of disability (PSMS score >24) was associatedwith a 4.3 times greater likelihood of MDR A. baumannii colonization (95% CI, 1.5–12.8; P<.006). Residents with both a feeding tube and urinary catheter were approximately 6 times more likely than residents with only one device to have MDR A. baumannii colonization (odds ratio, 6.1 [95% CI, 1.8–21.1]; P<.003). Diabetes was marginally associated with A. baumannii colonization (odds ratio, 2.9 [95% CI, 1.0–8.6]; P<.05).
Co-colonization With Other AROs
Weevaluated the frequency of colonizationwithR-GNB,MRSA, and VRE prior to or concurrent with the first incidence of a positive MDR A. baumannii culture (Table 2). Of the 25 cases, 22 (88%) had at least one incidence ofAROcolonization; Proteus mirabilis was the most prevalent organism among cases (56%), followed by VRE (48%), MRSA (44%), and Escherichia coli (44%). Of the 143 controls, 100 (70%) were colonized with at least one ARO during study follow-up; MRSA was the most prevalent organism among controls (43%), followed by VRE (25%), E. coli (23%), and P. mirabilis (17%). We were also interested in the frequency of co-colonization
(ie, colonizing the same body site on the same visit) between MDR A. baumannii and other R-GNBs. Sixteen cases (64%) were considered to be co-colonized. Of 48 swab samples that indicated co-colonization, 30 (63%) revealed a single co-colonizing organism (in addition to A. baumannii)and 17 (35%) revealed 2 co-colonizing organisms. In one swab sample,MDR A. baumannii and antibiotic-resistant P. mirabilis, E. coli,and Morganella morgannii were all isolated from the perianal area of one resident. Table 3 describes the frequency of co-colonizing organisms isolated from 4 different body sites.
Multivariable Model Examining A. baumannii Colonization
We explored the association among several predictors that were significant in bivariate models and MDR A. baumannii
(44%) involved recurrent A. baumannii colonization and 14 (56%) were colonized on a single visit (Figure 1). Cases and controls were similar in baseline characteristics including age, sex, comorbidity score, rate of recent hospitaliza- tions and antibiotic use, number of antibiotic classes used, and proportion of patients with urinary catheters (Table 1). Cases differed from controls in terms of functional disability, length of stay, total follow-up time, and presence of multiple indwelling devices. Cases had greater mean (SD) functional disability on average than controls (24.0 [3.8] vs 20.9 [4.5]), longer mean stays in theNHfacility before enrollment (657 vs 133 days), and longer mean follow-up (151 vs 55 days).
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