Characteristics and Outcomes of MRSA Pneumonia
Characteristics and Outcomes of MRSA Pneumonia
Concerns for the spread of CA-MRSA have led to widespread prescribing of empiric anti-MRSA therapy for patients presented to the hospital with pneumonia. Therefore our study aimed to identify differentiating characteristics for MRSA pneumonia and evaluate outcomes associated with empiric anti-MRSA therapy. Our study cohort involved an elderly population who differed significantly between those with MRSA vs non-MRSA pneumonia on risk factors, severity of presentation, and outcomes. Jung et al. published a retrospective study of community-onset MRSA vs non-MRSA pneumonia in a similar patient population and found comparable rates of mortality and length of stay, though antibiotic treatment was not evaluated.
Shorr et al. recently published a risk score assessment to aid clinicians in identifying those at risk for MRSA among patients presenting to the hospital with pneumonia. When applied to our study cohort, the scoring scheme had a positive predictive value of 68%, predicting those with non-MRSA pneumonia to have low risk and those with MRSA pneumonia to have high risk reasonably well. However, majority of patients fell within the medium risk group. Applying additional differentiating criteria identified from our study (e.g. history of MRSA infection or history of pneumonia within 1 year) reduced the number of non-MRSA patients in the medium risk group from 43% to 7%. Also, addition of the PSI class to the scoring scheme helped capture the small number of patients with MRSA pneumonia who were "falsely" deemed to have low risk as 89% of them had PSI IV or V compared to 42% in the non-MRSA group.
The 2005 Infectious Diseases Society of America guideline definition of HCAP has been criticized for its lack of specificity in identifying risks for resistant pathogens, leading to overprescription of broad-spectrum antibiotics. Furthermore, a study reported that guideline-adherent therapy in the management of nosocomial pneumonia, including MRSA HCAP was associated with increased mortality, hypothesized to be due to drug toxicities from aminoglycoside and colistin. Over one-third (37%) of the non-MRSA pneumonia patients were identified as HCAP by definition and 28% were given anti-MRSA therapy. By applying Shorr's scoring scheme, unnecessary prescribing of anti-MRSA therapy in low risk patients without MRSA pneumonia could have been reduced by 20%. Among the 11% of "low risk" patients who had MRSA pneumonia in our cohort, outcomes were similar regardless of receipt of empiric anti-MRSA therapy.
Vancomycin was the most commonly prescribed anti-MRSA therapy. Although 90% of MRSA pneumonia patients fit the definition of HCAP, the initiation rate of empiric anti-MRSA therapy was suboptimal at 68%, underscoring the need to improve early recognition of at-risk patients. By using the scoring system to guide empiric therapy, an additional 30% (40/134) of medium or high risk patients with MRSA pneumonia would have been promptly started on effective therapy. However, it is notable that a delay of 3 days in initiating anti-MRSA therapy did not lead to negative outcomes even among the subgroup of critically ill patients (APACHE II ≥18) with more severe presentation (n = 37), although, the number of patients in this subgroup is small and most received vancomycin therapy. It is possible that empiric therapy with non-vancomycin alternatives (ie. linezolid or ceftaroline) may alter outcomes in this subgroup and therefore deserves to be studied further.
This study has several limitations. First, our findings may not be applicable to all settings or patient populations due to the retrospective, single center design. We were unable to control for potentially confounding variables on outcomes such as antibiotic dosing and management of concurrent infections. MRSA grown from the respiratory culture may not represent the causative pathogen; however, we attempted to limit this possibility by including only patients who had MRSA as the sole pathogen from culture.
We included culture negative patients in the non-MRSA group in order to capture all those who presented to the hospital with pneumonia and received antibacterials for greater than 48 hours. Of the 106 patients who were culture negative in the non-MRSA group, 27/106 (25%) received empiric anti-MRSA therapy with the median duration of 4 days (IQR 1, 5.25). Outcomes were similar between those with negative cultures who received empiric anti-MRSA (n = 27) vs those did not (n = 79): favorable response in 25/27 (93%) vs 77/79 (97%), length of hospital stay (median: 6 vs 5 days). This data confirms that MRSA was not a likely pathogen in the culture-negative cohort and that anti-MRSA therapy was not needed in that group.
The definition of pneumonia used in this study was the CDC definition published in 2008. After our study had been completed, an updated definition was published in January 2014. Since the clinically defined pneumonia criteria remained the same and our first inclusion criteria was diagnosis of pneumonia regardless of culture results, we believe our screening criteria for inclusion into the study remains applicable. We acknowledge that the use of ICD-9 codes to identify patients can be inaccurate due to coding bias; however, we have attempted to rectify this limitation by including only patients who have met the clinical definition of pneumonia per the CDC criteria. Lastly, readmission rates could not be captured consistently as we do not have a closed healthcare system in which patients may be readmitted to the other institutions.
Discussion
Concerns for the spread of CA-MRSA have led to widespread prescribing of empiric anti-MRSA therapy for patients presented to the hospital with pneumonia. Therefore our study aimed to identify differentiating characteristics for MRSA pneumonia and evaluate outcomes associated with empiric anti-MRSA therapy. Our study cohort involved an elderly population who differed significantly between those with MRSA vs non-MRSA pneumonia on risk factors, severity of presentation, and outcomes. Jung et al. published a retrospective study of community-onset MRSA vs non-MRSA pneumonia in a similar patient population and found comparable rates of mortality and length of stay, though antibiotic treatment was not evaluated.
Shorr et al. recently published a risk score assessment to aid clinicians in identifying those at risk for MRSA among patients presenting to the hospital with pneumonia. When applied to our study cohort, the scoring scheme had a positive predictive value of 68%, predicting those with non-MRSA pneumonia to have low risk and those with MRSA pneumonia to have high risk reasonably well. However, majority of patients fell within the medium risk group. Applying additional differentiating criteria identified from our study (e.g. history of MRSA infection or history of pneumonia within 1 year) reduced the number of non-MRSA patients in the medium risk group from 43% to 7%. Also, addition of the PSI class to the scoring scheme helped capture the small number of patients with MRSA pneumonia who were "falsely" deemed to have low risk as 89% of them had PSI IV or V compared to 42% in the non-MRSA group.
The 2005 Infectious Diseases Society of America guideline definition of HCAP has been criticized for its lack of specificity in identifying risks for resistant pathogens, leading to overprescription of broad-spectrum antibiotics. Furthermore, a study reported that guideline-adherent therapy in the management of nosocomial pneumonia, including MRSA HCAP was associated with increased mortality, hypothesized to be due to drug toxicities from aminoglycoside and colistin. Over one-third (37%) of the non-MRSA pneumonia patients were identified as HCAP by definition and 28% were given anti-MRSA therapy. By applying Shorr's scoring scheme, unnecessary prescribing of anti-MRSA therapy in low risk patients without MRSA pneumonia could have been reduced by 20%. Among the 11% of "low risk" patients who had MRSA pneumonia in our cohort, outcomes were similar regardless of receipt of empiric anti-MRSA therapy.
Vancomycin was the most commonly prescribed anti-MRSA therapy. Although 90% of MRSA pneumonia patients fit the definition of HCAP, the initiation rate of empiric anti-MRSA therapy was suboptimal at 68%, underscoring the need to improve early recognition of at-risk patients. By using the scoring system to guide empiric therapy, an additional 30% (40/134) of medium or high risk patients with MRSA pneumonia would have been promptly started on effective therapy. However, it is notable that a delay of 3 days in initiating anti-MRSA therapy did not lead to negative outcomes even among the subgroup of critically ill patients (APACHE II ≥18) with more severe presentation (n = 37), although, the number of patients in this subgroup is small and most received vancomycin therapy. It is possible that empiric therapy with non-vancomycin alternatives (ie. linezolid or ceftaroline) may alter outcomes in this subgroup and therefore deserves to be studied further.
Limitations
This study has several limitations. First, our findings may not be applicable to all settings or patient populations due to the retrospective, single center design. We were unable to control for potentially confounding variables on outcomes such as antibiotic dosing and management of concurrent infections. MRSA grown from the respiratory culture may not represent the causative pathogen; however, we attempted to limit this possibility by including only patients who had MRSA as the sole pathogen from culture.
We included culture negative patients in the non-MRSA group in order to capture all those who presented to the hospital with pneumonia and received antibacterials for greater than 48 hours. Of the 106 patients who were culture negative in the non-MRSA group, 27/106 (25%) received empiric anti-MRSA therapy with the median duration of 4 days (IQR 1, 5.25). Outcomes were similar between those with negative cultures who received empiric anti-MRSA (n = 27) vs those did not (n = 79): favorable response in 25/27 (93%) vs 77/79 (97%), length of hospital stay (median: 6 vs 5 days). This data confirms that MRSA was not a likely pathogen in the culture-negative cohort and that anti-MRSA therapy was not needed in that group.
The definition of pneumonia used in this study was the CDC definition published in 2008. After our study had been completed, an updated definition was published in January 2014. Since the clinically defined pneumonia criteria remained the same and our first inclusion criteria was diagnosis of pneumonia regardless of culture results, we believe our screening criteria for inclusion into the study remains applicable. We acknowledge that the use of ICD-9 codes to identify patients can be inaccurate due to coding bias; however, we have attempted to rectify this limitation by including only patients who have met the clinical definition of pneumonia per the CDC criteria. Lastly, readmission rates could not be captured consistently as we do not have a closed healthcare system in which patients may be readmitted to the other institutions.
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