Intended for healthcare professionals

Rapid response to:

Education And Debate

Problems in assessing rates of infection with methicillin resistant Staphylococcus aureus

BMJ 2005; 331 doi: (Published 27 October 2005) Cite this as: BMJ 2005;331:1013

Rapid Response:

Size matters

David J Spiegelhalter draws attention to the statistical difficulties
is assessing rates of infection with methicillin resistant Staphylococcus
aureus (MRSA) and demonstrates the large, apparently random, variation in
these rates[1].

The Government data he analyses[2] are presented in a way that seeks
to explain some of the variation by categorising the NHS Trusts that
operate hospitals into “single specialty” Trusts with low rates of
infection, “specialist” Trusts which include these with the highest rates
and “general acute” Trusts with intermediate levels. The specialist
Trusts have high rates because, it is suggested, they receive patients by
transfer who import infection. This is unlikely to be a complete
explanation; single specialty Trusts also receive such patients and the
Trusts described as “specialist” are usually major teaching hospitals
whose tertiary specialist functions are carried out alongside general
acute functions.

There is in fact a clear relationship with the size of the Trust and
its location in London or the Provinces. Using the 2003-2004 data and
excluding the single specialty Trusts, for provincial Trusts the
correlation coefficient is 0.243532465 for MRSA bacteraemia rate against
bed days (n=128, p<0.01) and for London Trusts it is 0.42544808 (n=27,

These correlations may reflect the diseconomies of scale associated
with large hospitals[3]. However, the unit of assessment of the rate of
MRSA bacteraemia is the NHS Trust and the largest Trusts were formed by
the merger of smaller ones. They therefore operate several hospitals. It
is most likely that the circumstances, largely concerned with finance and
staffing, that led to the meger of Trusts are the circumstances that lead
to higher rates of MRSA bacteraemia.

1. Spiegelhalter DJ, Problems in assessing rates of infection with
methicillin resistant Staphylococcus aureus BMJ 2005;331:1013-1015

2. Department of Health. Mandatory bacteraemia surveillance
scheme—MRSA bacteraemia by NHS trust: April 2001-March 2004.

3. Hospital volume and health care outcomes, costs and patient
access. Effective Health Care, University of York, 2 (8) 1996

Competing interests:
None declared

Competing interests: No competing interests

07 November 2005
S. Michael Crawford
Consultant Medical Oncologist
Airedale General Hospital, Skipton Road, Steeton, Keighley, West Yorkshire, BD20 6TD