Scottish Surveillance of Healthcare Associated Infection Programme (SSHAIP)

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SSHAIP

Weekly Report Articles

09 January 2013

Quarterly report on the surveillance of Staphylococcus aureus bacteraemias in Scotland, July - September 2012

Executive summary

  • This report provides a summary of Staphylococcus aureus (S. aureus) bacteraemia data for the third quarter of 2012 (July to September 2012) in 15 NHS boards. This includes data on both meticillin resistant S. aureus (MRSA) and meticillin sensitive S. aureus (MSSA) bacteraemias; these data are compared to previous quarters.
  • Altogether, 367 new S. aureus bacteraemia cases were reported to Health Protection Scotland (HPS) during this period. This is a decrease of 4.2% compared with the same quarter last year (July to September 2011) when 383 cases were reported and is the lowest number of cases reported since the start of the mandatory S. aureus bacteraemia surveillance programme.
  • In the last year, October 2011 to September 2012, 1538 episodes of S. aureus bacteraemia were reported to HPS. This represents a decrease of 7.9% on the previous year, October 2010 to September 2011, when 1670 episodes were reported.
  • The overall rate of S. aureus bacteraemias for Scotland was 29.3 S. aureus bacteraemia cases per 100 000 acute occupied bed days (AOBDs). This represents a decrease of 2.8% compared to the previous quarter (from 30.2 to 29.3). It is also the lowest equal S. aureus bacteraemia rate reported since the start of the mandatory S. aureus bacteraemia surveillance programme.
  • In comparison with the same quarter of 2011, the overall S. aureus bacteraemia rate for Scotland has decreased by 4.2%, from 30.6 to 29.3 per 100 000 AOBDs.
  • The number and rate of MRSA bacteraemias reported in the period July to September 2012 was the lowest of any quarter reported since the start of this surveillance programme with 33 cases and 2.6 per 100 000 AOBDs.
  • During this quarter, the S. aureus, MRSA and MSSA bacteraemia rates in all NHS boards were within or below the 95% confidence limits on the funnel plots.
  • The national surveillance programme reports on S. aureus bacteraemias arising three months or longer before publication of this report. It remains essential therefore that it is complemented by more contemporaneous local monitoring of S. aureus bacteraemias by NHS boards themselves.

1. Introduction

Staphylococcus aureus (S. aureus) is a gram positive bacterium which colonises the nasal cavity of about 30% of the healthy population. Although this colonisation is usually harmless, S. aureus may cause serious infections. These infections are commonly associated with healthcare interventions, often because of failures to implement infection prevention methods. As a result, both meticillin sensitive and meticillin resistant S. aureus (MSSA and MRSA) remain endemic in many UK hospitals, causing a range of infections. Amongst the most serious of these are bacteraemias.

The Health Protection Scotland (HPS) S. aureus bacteraemia surveillance programme monitors the occurrence of S. aureus bacteraemias amongst all patients in Scotland. It includes S. aureus bacteraemias occurring in patients who have been in contact with the healthcare system (in both acute and non-acute hospitals, as well as in primary care settings) and those who have acquired S. aureus bacteraemias in the community, without any healthcare contacts.

The surveillance programme in Scotland includes data on both MRSA and MSSA bacteraemias. Many other countries restrict surveillance of S. aureus bacteraemias to those caused only by MRSA.

This quarterly report is concerned with the incidence of S. aureus bacteraemias in Scotland and within individual NHS boards. HPS publishes an annual report on S. aureus bacteraemias in Scotland.1 This report contains further analyses of S. aureus data, including trends in S. aureus bacteraemias.

The quarterly S. aureus bacteraemia data produced by HPS are based on interim data for both bed occupancy and incident S. aureus bacteraemias. These data are subject to revision as finalised data become available. Therefore there may occasionally be minor numeric discrepancies between reports, reflecting the availability of such updated data.

Finally, it should be noted that the national S. aureus bacteraemia surveillance programme contains information on bacteraemias which occurred at least three months and possibly up to six months, before this report was published. It is therefore imperative that NHS boards have their own local S. aureus bacteraemia surveillance systems to alert local teams to the need for possible early action. This surveillance report does not replace local monitoring of S. aureus bacteraemias by NHS boards' infection control teams.

2. Methods

Full details of the data source and the definitions used to generate this report are published on the HPS website.2 The scaling factor used in reporting incidence rates in this report is now 'per 100 000 bed days' instead of the previously used 'per 1000 bed days'.

Identification of outliers

Comparisons of NHS boards' S. aureus bacteraemia, MRSA and MSSA rates are shown using funnel plots.3 These are scatterplots of the observed S. aureus bacteraemia events, measured as rates, plotted against their precision, indicated by the number of acute occupied bed days. The plots in this report show the upper and lower 95% confidence limits as curved lines. If an individual NHS board's rate was outwith the 95% confidence limit, this would be regarded as an outlier, suggesting that that NHS board had a S. aureus bacteraemia rate which was significantly different to other NHS boards.

Analysis of trends

Analysis of trends in the rates of S. aureus bacteraemias was carried out within an over-dispersed Poisson regression model. The regression included terms for NHS board, year and quarter. Model checking was performed using residual plots and these demonstrated that the model assumptions were reasonable. Trend analyses were based on the latest 30 quarters of data.

3. Results

3.1. Surveillance data

During the period July to September 2012, HPS received reports of S. aureus bacteraemias from 21 diagnostic laboratories in 13 NHS boards as well as from the Scottish MRSA Reference Laboratory.

The total number of S. aureus bacteraemia cases identified in Scotland was 367, of which 33 were MRSA bacteraemias (9.0%) and 334 were MSSA bacteraemias (91.0%) as shown in Table 1. The total numbers of S. aureus bacteraemias in Scotland decreased by 4.4% compared with the previous quarter (April to June 2012) when 384 S. aureus bacteraemia cases were recorded. In that quarter, 54 (14.1%) were due to MRSA and 330 (85.9%) to MSSA.

The overall S. aureus bacteraemia rate for Scotland during this quarter, July to September 2012, was 29.3 per 100 000 AOBDs. This is a 4.2% decrease on the overall rate for Scotland from the corresponding quarter in the previous year. This represents the lowest number and lowest equal rate of S. aureus bacteraemias since the incorporation of MSSA bacteraemias into this surveillance system.

This quarter, July to September 2012, had the lowest number and rate of MRSA bacteraemias of any quarter since the start of the mandatory S. aureus bacteraemia surveillance programme.

The total numbers of S. aureus bacteraemia cases and the S. aureus bacteraemia rates for each NHS board during this quarter, July to September 2012, are listed in Table 2, which also indicates the numbers and rates of S. aureus bacteraemia in the preceding year, July 2011 to June 2012. All historic data of cases, AOBDs and S. aureus bacteraemia rates for each NHS board are included in the online appendix 2 (at http://www.hps.scot.nhs.uk/haiic/sshaip/publicationsdetail.aspx?id=30248).

3.2. Comparison of NHS boards' rates

Figure 1a shows a funnel plot of S. aureus bacteraemia rates for individual NHS boards for this quarter, Figures 1b and 1c show similar funnel plots for MRSA and MSSA bacteraemias, respectively. They show that none of the S. aureus bacteraemia rates in NHS boards was above the upper 95% confidence limit.

3.3. Identification of trends

The overall quarterly rates for Scotland are shown in Figure 2. A Poisson regression analysis of trends in the S. aureus bacteraemia rate between April 2005 and September 2012 was carried out. This showed a significant year on year reduction in the rate of S. aureus bacteraemias for NHSScotland. This was estimated to be 7.6% per year (95% CI: 6.4% to 8.7%). The rate of MRSA bacteraemias also showed a significant year on year reduction estimated at 21.0% (95% CI: 18.7% to 23.4%) as did the rate of MSSA bacteraemias which was estimated to be 2.1% per year (95% CI: 0.6% to 3.6%). The S. aureus, MRSA and MSSA rates for Scotland are shown as rolling four-quarter rates in Figure 3. Similar rolling four-quarter rates for all NHS boards are included in the online appendix 1 (at http://www.hps.scot.nhs.uk/haiic/sshaip/publicationsdetail.aspx?id=30248).

4. Discussion

This report covers S. aureus bacteraemia surveillance data for the period July to September 2012. Unlike the national surveillance schemes in many other countries, it includes both MRSA and MSSA bacteraemias.

A number of important caveats associated with the data in this report must be highlighted. This includes the use of estimated denominator data, numerator data being self-reported by participating laboratories, the inclusion of some S. aureus bacteraemias which may be community-acquired and only diagnosed on admission to hospital and possible multiple reports on patients due to non-compliance with treatment or treatment failure.

Altogether, 367 cases of S. aureus bacteraemia were identified during this period, a decrease of 4.4% compared with the total number from the previous quarter.

The proportion of S. aureus bacteraemias which were meticillin resistant during the quarter July to September 2012 was 9.0%. The 2011 figures for the Europe, published in the Antimicrobial resistance surveillance in Europe 2011 report show that the proportion of S. aureus bacteraemia which were meticillin resistant was 16.7%.4

Funnel plot analysis was used to examine variations in NHS board S. aureus bacteraemia rates. No NHS boards were above the upper 95% confidence interval in the S. aureus bacteraemia funnel plots. The funnel plots do not account for differences in the clinical activities performed in different NHS boards. Therefore, care must be taken in interpreting these graphs, particularly for the NWTC which is a small board providing specialist services.

The national S. aureus bacteraemia rate per 100 000 AOBDs during the period April 2005 to September 2012 showed a significant downward trend and this was estimated as a reduction of 7.6% per year (95% CI: 6.4% to 8.7%).

Acknowledgements

Health Protection Scotland is grateful to all of the microbiologists throughout Scotland who provided the S. aureus bacteraemia data for this report and helped in its preparation, staff at the Scottish MRSA Reference Laboratory and to the Information Services Division of National Services Scotland for providing the hospital activity data. Our thanks also go to Information Services Division of National Services Scotland for their statistical support.

References

  1. Health Protection Scotland. The Annual Surveillance of Healthcare Associated Infections Report January - December 2011 [online]. Available: http://www.hps.scot.nhs.uk/haiic/sshaip/publicationsdetail.aspx?id=51462 [accessed 22 November 2012]. 2012.
  2. Health Protection Scotland. Protocol for the Scottish Mandatory Surveillance Programme for Staphylococcus aureus bacteraemia [online]. Available: http://www.hps.scot.nhs.uk/haiic/sshaip/guidelinedetail.aspx?id=30697 [accessed 22 November 2012]. 2011.
  3. Spiegelhalter D. Funnel plots for comparing institutional performance. Statistics in Medicine. 2005;24:1185-202.
  4. European Centre for Disease Prevention and Control (ECDC). Antimicrobial resistance surveillance in Europe 2011 [online]. Available: http://ecdc.europa.eu/en/publications/Publications/Forms/ECDC_DispForm.aspx?ID=998 [accessed 22 November 2012]. 2011:1-209
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Author(s): Prepared by: Health Protection Scotland Vol: 47 No: 02 Year: 2013 Page:

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