Trends in Staphylococcus aureus bacteraemia and impacts of infection control practices including universal MRSA admission screening in a hospital in Scotland, 2006-2010: retrospective cohort study and time-series intervention analysis

Abstract

Objectives: To describe secular trends in Staphylococcus aureus bacteraemia (SAB) and to assess the impacts of infection control practices, including universal methicillin-resistant Staphylococcus aureus (MRSA) admission screening on associated clinical burdens.

Design: Retrospective cohort study and multivariate time-series analysis linking microbiology, patient management and health intelligence databases.

Setting: Teaching hospital in North East Scotland.

Participants: All patients admitted to Aberdeen Royal Infirmary between 1 January 2006 and 31 December 2010: n=420 452 admissions and 1 430 052 acute occupied bed days (AOBDs).

Intervention: Universal admission screening programme for MRSA (August 2008) incorporating isolation and decolonisation. PRIMARY AND SECONDARY MEASURES: Hospital-wide prevalence density, hospital-associated incidence density and death within 30 days of MRSA or methicillin-sensitive Staphylococcus aureus (MSSA) bacteraemia.

Results: Between 2006 and 2010, prevalence density of all SAB declined by 41%, from 0.73 to 0.50 cases/1000 AOBDs (p=0.002 for trend), and 30-day mortality from 26% to 14% (p=0.013). Significant reductions were observed in MRSA bacteraemia only. Overnight admissions screened for MRSA rose from 43% during selective screening to >90% within 4 months of universal screening. In multivariate time-series analysis (R(2) 0.45 to 0.68), universal screening was associated with a 19% reduction in prevalence density of MRSA bacteraemia (-0.035, 95% CI -0.049 to -0.021/1000 AOBDs; p<0.001), a 29% fall in hospital-associated incidence density (-0.029, 95% CI -0.035 to -0.023/1000 AOBDs; p<0.001) and a 46% reduction in 30-day mortality (-15.6, 95% CI -24.1% to -7.1%; p<0.001). Positive associations with fluoroquinolone and cephalosporin use suggested that antibiotic stewardship reduced prevalence density of MRSA bacteraemia by 0.027 (95% CI 0.015 to 0.039)/1000 AOBDs. Rates of MSSA bacteraemia were not significantly affected by screening or antibiotic use.

Conclusions: Declining clinical burdens from SAB were attributable to reductions in MRSA infections. Universal admission screening and antibiotic stewardship were associated with decreases in MRSA bacteraemia and associated early mortality. Control of MSSA bacteraemia remains a priority.

Figure 1

Study overview in accordance with the ORION (Outbreak Reports and Intervention Studies Of Nosocomial infection) statement. *‘4C’ antibiotics are clindamycin, ciprofloxacin (all fluoroquinolones), cephalosporins (all generations), co-amoxiclav. AOBDs, acute occupied bed days; ICD, infection control doctor; ICN, infection control nurse; ICU, intensive care unit; MRSA, methicillin-resistant Staphylococcus aureus; SAB, Staphylococcus aureus bacteraemia; WTE, whole-time equivalents calculated as 37.5 h/week×52 weeks =1950 h/year.

Figure 2

Rates of Staphylococcus aureus bacteraemia by age group, length of stay and days from admission. p<0.01 for all linear regression lines. Note logarithmic scale for length of stay. Linear trend fitted after logarithmic transformation. MRSA, methicillin-resistant Staphylococcus aureus; MSSA, methicillin-sensitive Staphylococcus aureus.

Figure 3

Secular trends in prevalence density and all-cause 30-day mortality after Staphylococcus aureus bacteraemia by methicillin resistance and admitting department (MRSA only). Data aggregated in 3-month blocks. Lines represent results of trend analysis, using Poisson regression with time (month) as sole explanatory variable. AOBDs, acute occupied bed days; ICU, intensive care unit; MRSA, methicillin-resistant Staphylococcus aureus; MSSA, methicillin-sensitive Staphylococcus aureus.

Figure 4

Adherence to methicillin-resistant Staphylococcus aureus (MRSA) admission testing during universal surveillance (August 2008 to December 2010). Special study period (February 2010 to August 2010) involved a trial of axillae and groin swabs.

Figure 5

Observed trends and multivariate transfer model predictions (sum of lagged explanatory variables) for prevalence density, hospital-associated (HA) incidence density, 30-day mortality in methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia and % Staphylococcus aureus bacteraemia (SAB) involving MRSA. CL, confidence limit.