Patients were included if they met all of the following criteria: age 18 years or older, with no upper age limit; treated for a LRTI, identified by ICD-10 codes (J13-22, J44, J85); treated with empiric antibiotics active against MRSA, specifically vancomycin or linezolid; and treated at the Hospital of Central Connecticut (HOCC) or Hartford Hospital (HH) between July 1, 2018, and June 30, 2019, inclusive. Patients were excluded if they met 1 or more of the following criteria: age less than 18 years old; admitted for 48 hours or fewer or discharged from the emergency department; not treated at either facility; treated before July 1, 2018, or after June 30, 2019; treated for ventilator-associated pneumonia; received anti-MRSA therapy within 30 days prior to admission; or treated for a concurrent bacterial infection requiring anti-MRSA therapy.
Outcome Measures
The primary outcome was anti-MRSA days of therapy (DOT) in patients who underwent MRSA PCR testing compared to patients who did not undergo MRSA PCR testing. A subgroup analysis was completed to compare anti-MRSA DOT within patients in the MRSA PCR group. Patients in the subgroup were categorized by appropriate or inappropriate utilization, and anti-MRSA DOT were compared between these groups. Secondary outcomes that were evaluated included length of stay (LOS), 30-day readmission rate, and incidence of acute kidney injury (AKI). Thirty-day readmission was defined as admission to HOCC, HH, or any institution within Hartford HealthCare within 30 days of discharge. AKI was defined as an increase in serum creatinine by ≥ 0.3 mg/dL in 48 hours, increase ≥ 1.5 times baseline, or a urine volume < 0.5 mL/kg/hr for 6 hours.
Statistical Analyses
The study was powered for the primary outcome, anti-MRSA DOT, with a clinically meaningful difference of 1 day. Group sample sizes of 240 in the MRSA PCR group and 160 in the no MRSA PCR group would have afforded 92% power to detect that difference, if the null hypothesis was that both group means were 4 days and the alternative hypothesis was that the mean of the MRSA PCR group was 3 days, with estimated group standard deviations of 80% of each mean. This estimate used an alpha level of 0.05 with a 2-sided t-test. Among those who received a MRSA PCR test, a clinically meaningful difference between appropriate and inappropriate utilization was 5%.
Descriptive statistics were provided for all variables as a function of the individual hospital and for the combined data set. Continuous data were summarized with means and standard deviations (SD), or with median and interquartile ranges (IQR), depending on distribution. Categorical variables were reported as frequencies, using percentages. All data were evaluated for normality of distribution. Inferential statistics comprised a Student’s t-test to compare normally distributed, continuous data between groups. Nonparametric distributions were compared using a Mann-Whitney U test. Categorical comparisons were made using a Fisher’s exact test for 2×2 tables and a Pearson chi-square test for comparisons involving more than 2 groups.
Since anti-MRSA DOT (primary outcome) and LOS (secondary outcome) are often non-normally distributed, they have been transformed (eg, log or square root, again depending on distribution). Whichever native variable or transformation variable was appropriate was used as the outcome measure in a linear regression model to account for the influence of factors (covariates) that show significant univariate differences. Given the relatively small sample size, a maximum of 10 variables were included in the model. All factors were iterated in a forward regression model (most influential first) until no significant changes were observed.