Table 3.
Association Between Hospital, Case-mix, and Geographic Characteristics and Antibiotic Days of Therapy in Adjusted Models
| Characteristic | Antibiotic Category (n = 576) | ||||||
|---|---|---|---|---|---|---|---|
| All | BL/BLIs | 3rd/4th-Generation Cephalosporins | Glycopeptides | Carbapenems | Anti-PSA Agents | Anti-MRSA Agents | |
| Facility characteristic | |||||||
| Teaching hospital | 0.99 (.95–1.02) | 0.97 (.89–1.06) | 0.92 (.85–.98)* | 0.98 (.92–1.04) | 1.03 (.90–1.17) | 0.91 (.86–.97)* | 1.04 (.99–1.09) |
| Urban | 0.99 (.96–1.03) | NA | 0.93 (.87–.99)* | NA | NA | 0.97 (.92–1.03) | NA |
| Bed sizea | 1.00 (.99–1.01) | NA | 0.99 (.97–1.02) | NA | NA | 1.03 (1.01–1.04)* | NA |
| US census division | |||||||
| East North Central | 0.98 (.94–1.03) | 0.96 (.86–1.06) | 1.01 (.93–1.10) | 1.03 (.96–1.10) | 1.03 (.88–1.21) | 0.98 (.92–1.05) | 0.95 (.90–1.01) |
| East South Central | 0.99 (.93–1.06) | 0.94 (.81–1.10) | 0.97 (.86–1.09) | 0.97 (.88–1.08) | 1.28 (1.02–1.62)* | 0.96 (.87–1.07) | 1.03 (.95–1.12) |
| Middle Atlantic | 0.82 (.77–.86)* | 0.74 (.65–.84)* | 0.93 (.84–1.03) | 0.95 (.87–1.03) | 0.81 (.67–.99)* | 0.82 (.75–.89)* | 0.82 (.76–.88)* |
| Mountain | 0.97 (.90–1.05) | 1.07 (.88–1.29) | 0.90 (.77–1.04) | 0.92 (.81–1.05) | 0.48 (.36–.64)* | 0.89 (.79–1.01) | 0.91 (.82–1.01) |
| New England | 0.85 (.77–.93)* | 0.68 (.54–.86)* | 1.10 (.91–1.32) | 0.96 (.82–1.13) | 0.41 (.29–.59)* | 0.69 (.59–.81)* | 0.92 (.80–1.05) |
| Pacific | 0.94 (.89–.98) | 0.86 (.77–.97)* | 1.06 (.97–1.16) | 0.91 (.83–.98)* | 0.80 (.67–.97)* | 0.82 (.76–.89)* | 0.90 (.84–.96)* |
| West North Central | 0.98 (.93–1.04) | 0.90 (.78–1.03) | 1.00 (.89–1.11) | 0.97 (.88–1.07) | 1.04 (.84–1.29) | 0.99 (.90–1.09) | 0.93 (.86–.99)* |
| West South Central | 1.04 (.99–1.10) | 0.98 (.86–1.10) | 0.98 (.89–1.08) | 0.99 (.91–1.07) | 1.64 (1.36–1.98)* | 1.07 (.98–1.16) | 0.99 (.92–1.06) |
| South Atlantic | Ref | Ref | Ref | Ref | Ref | Ref | Ref |
| Percentage of PD with a bacterial infection diagnosisb | 1.33 (1.29–1.38)* | 1.47 (1.36–1.59)* | 1.49 (1.40–1.59)* | 1.45 (1.37–1.53)* | 1.66 (1.47–1.88)* | 1.51 (1.43–1.59)* | 1.36 (1.30–1.41)* |
| Percentage of PD in ICUs | NA | 1.009 (1.004–1.013)* | NA | 1.003 (1.000–1.007)* | 1.006 (.998–1.013) | 1.005 (1.002–1.008)* | 1.002 (.999–1.004) |
| Mean hospital patient age, yc | |||||||
| < 50 | 0.94 (.79–1.11) | 1.32 (.85–2.06) | 1.22 (.86–1.74) | 1.65 (1.21–2.24)* | 1.15 (.76–1.75) | 1.18 (.89–1.57) | 1.13 (.90–1.43) |
| 50 to < 60 | Ref | Ref | Ref | Ref | Ref | Ref | Ref |
| 60 to < 70 | 1.00 (.97–1.04) | 0.96 (.88–1.04) | 0.95 (.89–1.01) | 0.97 (.92–1.03) | 0.95 (.84–1.07) | 1.00 (.94–1.05) | 0.96 (.91–1.00) |
| ≥ 70 | 0.95 (.79–1.16) | 0.97 (.62–1.53) | 0.80 (.55–1.17) | 0.87 (.63–1.20) | 0.88 (.60–1.29) | 1.06 (.78–1.43) | 0.94 (.72–1.22) |
| Age-stratified effectsd | |||||||
| Hospital mean Elixhauser comorbidity index scoree | |||||||
| < 50 | 0.85 (.75–.98)* | 1.24 (.87–1.76) | 1.18 (.91–1.53) | 1.52 (1.20–1.92)* | f | 0.91 (.73–1.14) | 0.99 (.83–1.18) |
| 50 to < 60 | 0.95 (.91–.99)* | 0.96 (.86–1.08) | 1.04 (.96–1.13) | 0.95 (.88–1.02) | f | 0.91 (.85–.98)* | 0.99 (.93–1.05) |
| 60 to < 70 | 0.96 (.92–1.01) | 0.92 (.82–1.03) | 1.12 (1.03–1.23)* | 1.00 (.92–1.08) | f | 0.98 (.91–1.06) | 1.02 (.96–1.08) |
| ≥ 70 | 0.92 (.73–1.17) | 0.69 (.40–1.20) | 1.39 (.87–2.21) | 0.87 (.58–1.31) | f | 0.80 (.55–1.17) | 0.75 (.54–1.05) |
| Hospital CMI | |||||||
| < 50 | 1.89 (1.47–2.42)* | 2.36 (1.25–4.45)* | 2.16 (1.41–3.31)* | 3.71 (2.45–5.62)* | 6.10 (2.60–14.31)* | 2.63 (1.77–3.93)* | 2.97 (2.13–4.14)* |
| 50 to < 60 | 1.12 (1.02–1.22)* | 0.95 (.74–1.21) | 1.32 (1.15–1.51)* | 1.72 (1.49–2.00)* | 1.58 (1.07–2.33)* | 1.25 (1.10–1.42)* | 1.34 (1.20–1.51)* |
| 60 to < 70 | 1.11 (1.01–1.23)* | 1.09 (.86–1.39) | 1.09 (.90–1.32) | 1.71 (1.45–2.02)* | 1.66 (1.15–2.42)* | 0.99 (.84–1.17) | 1.37 (1.21–1.56)* |
| ≥ 70 | 1.25 (1.01–1.56)* | 1.66 (.96–2.87) | 1.15 (.78–1.68) | 1.94 (1.35–2.80)* | 2.03 (.88–4.72) | 1.47 (1.02–2.10)* | 1.63 (1.23–2.17)* |
Data are presented as adjusted incidence rate ratio (95% confidence interval). Associations were evaluated using multivariable negative binomial regression models with an offset equal to the natural log of total patient-days per facility. Included variables varied by antibiotic outcome.
Abbreviations: anti-MRSA, agents with targeted activity against methicillin-resistant Staphylococcus aureus; anti-PSA, agents with targeted activity against Pseudomonas spp; BL/BLI, β-lactam/β-lactamase inhibitor combination; CMI, case-mix index; ICU, intensive care unit; NA, variable was not included in a given model because in unadjusted analysis its P value was ≥ .10; PD, patient-days; US, United States.
aBed size was coded ordinally, categories 1–6. Each 1-unit increase represents an additional 100 beds, up to ≥ 500 (reference ≤ 99 beds).
bAgency for Healthcare Research and Quality (AHRQ) bacterial infection diagnosis codes were used to ascertain whether a patient’s primary International Classification of Diseases, Tenth Revision diagnosis was infection-related (AHRQ, 2019). Inpatient-days for encounters with a primary infection diagnosis were treated as “bacterial infection” patient-days. Each adjusted incidence rate ratio (aIRR) represents the average effect of a 10 percentage point increase in a hospital’s percentage of bacterial infection patient-days, holding other factors constant.
cHospital mean patient age was stratified into 4 categories to increase interpretability of interactions by age (mean Elixhauser score and CMI). aIRR estimates for each age strata reflect the adjusted effect of age, relative to the reference category of 50 to < 60 years, at the cohort’s mean Elixhauser score and mean CMI values (3.2 and 1.49, respectively).
dBiologically plausible interactions were retained in final multivariable models if at least 1 age strata’s P value was < .10 and at least 1 other strata’s P value was ≥ .10, or if effect estimates across age strata differed by > 10% in analyses including only the main and interaction effects. Each aIRR estimate reflects the adjusted average effect of a 1-unit increase in a given variable (CMI or mean Elixhauser comorbidity index score) in hospitals with this average patient age, holding other factors constant.
eElixhauser comorbidity categories were modified to include primary diagnoses, in addition to secondary diagnoses. Elixhauser scores represent unweighted Elixhauser comorbidity sums (1 point per comorbidity). An average patient Elixhauser score was calculated for each hospital.
fCarbapenems were the sole outcome where unadjusted analyses did not support an interaction between mean Elixhauser score and average patient age; thus, mean Elixhauser score was not age-stratified in the final adjusted model for carbapenem use. The aIRR for mean Elixhauser score in the adjusted carbapenem model was 0.83 (95% confidence interval, .73–.94; P = .004).
*Significant at an α level of P < .05 (and bolded).