Problems in assessing rates of infection with methicillin resistant Staphylococcus aureus

Abstract

Chance variability makes it impossible to assess reliably whether individual trusts are meeting annual targets for reduction in the risk of MRSA infection

Fig 1

MRSA cases per 1000 bed days in 173 NHS acute trusts in 2003-4. The two funnels indicate where 95% and 99.8% of trusts would lie by chance alone, assuming no underlying variability within each type of trust. These boundaries correspond to 2 and 3 standard deviation control limits, widened by 33% to allow for over-dispersion. Hospitals with no cases observed in 2003-4 (Birmingham Women's Hospital and Moorfields Eye Hospital) are not plotted

Fig 2

Ratio of MRSA rates in 2003-4 to those in 2002-3 in 169 NHS acute trusts plotted against (left) the average number of cases each year, with 95% and 99.8% control limits around an assumption of no change (ratio =1) and (right) the rate in 2002-3. Four hospitals with no cases observed in one or both years are not plotted

Fig 3

Funnel plots for a true annual risk reduction of 0% (left) and 20% (right). Trusts with data in the shaded area would achieve the 20% annual target. M1 corresponds to a median trust (32 cases a year) that achieves a significant reduction. M2 corresponds to a median trust that is just compatible with the target reduction