The number of infections caused by methicillin resistant Staphylococcus aureus (MRSA) is increasing every year in England and Wales.1,2 These infections are more difficult to treat than others because of the resistance of the bacterium to first line antibiotics. The impact of these infections on mortality has been unknown; data on the mortality caused by MRSA infections is not routinely available because the international classification of diseases (ICD) has no code for these infections. The evidence that the infections are associated with a higher mortality than methicillin sensitive S aureus infections is equivocal.1 We used death certificates to examine the evidence that mortality due to MRSA and staphylococcal infections in England and Wales is increasing.
Methods and results
In 1993 redevelopment of the processing systems for death registrations in England and Wales enabled death registration data to be analysed by all conditions mentioned on death certificates (rather than by the final underlying cause alone).3 ICD-9 (ICD, 9th revision) was in use during the period of this study.
We examined all death registrations in the Office for National Statistics database with ICD-9 codes 05.0, 08.4, 038.1, 041.1, 320.3, and 482.4, indicating staphylococcal infection, on any part of the death certificate for deaths that occurred between 1 January 1993 and 31 December 1998. We manually identified the inclusion of MRSA by noting the text entered on each line of the death certificate, including underlying and contributory conditions. We calculated age group specific annual mortality using mid-year population estimates from the Office for National Statistics.
MRSA was mentioned on 1387/6723 (20.6%) death certificates that included an ICD-9 code for staphylococcal infection (table). The percentage of certificates mentioning MRSA increased from 7.5% in 1993 to 25.0% in 1998. The final underlying causes of death indicated by death certificates that also mentioned MRSA included infections, neoplasms, and disease of nearly every system of the body.
The number of certificates mentioning staphylococcal infection and the number of deaths with staphylococcal infection as the underlying cause increased each year. Each year, a similar proportion of certificates mentioned staphylococcal infection as the underlying cause of death; in these certificates, the proportion mentioning MRSA increased from 8% in 1993 to 44% in 1998 (13/156 v 114/258). MRSA accounted for all of the increase in deaths due to staphylococcal infection in this period: MRSA in staphylococcal septicaemia increased from 3% to 28% (3/87 v 37/134), staphylococcal pneumonia from 13% to 44% (6/47 v 24/54), and unspecified bacterial infection, staphylococcus from 19% to 83% (4/21 v 53/64).
In certificates mentioning MRSA where staphylococcal infection was the final underlying cause of death, mortality was higher in men and in older people. For 86% of the certificates, the age of the person who died was over 64. In 1998 mortality ranged from 0.4 per 100 000 for women aged 45-64 to 14.8 per 100 000 for men over 84.
Comment
Infections due to MRSA seem to be an increasing cause of mortality in England and Wales. Improved reporting is unlikely to explain the increase. The greatest rise in MRSA occurred for deaths in which invasive staphylococcal infection was given as the final underlying cause, so antimicrobial resistance probably influenced the success of medical management.
Our study highlights the limitations of using routine mortality data for monitoring the impact of MRSA. There is no code for this infection in either ICD-9 or ICD-10. The Office for National Statistics could introduce routine automated searches of computerised text or assign one of the unused ICD-10 “U” codes available for special studies to MRSA.
Further improvements in surveillance and control of healthcare associated infection and mortality should be a priority if MRSA related deaths are to be prevented.4 Recent initiatives, such as the requirement since April 2001 for all NHS trusts to report S aureus bacteraemia, will help towards achieving this goal.5
Table.
Death registrations with staphylococcal ICD-9 codes by year of death. Results are numbers (percentages)
| Year of death
|
1993
|
1994
|
1995
|
1996
|
1997
|
1998
|
Total 1993-8
|
|---|---|---|---|---|---|---|---|
| No of certificates with any mention of MRSA | 47 (7.5) | 88 (11.4) | 187 (18.3) | 290 (22.8) | 377 (26.2) | 398 (25.0) | 1387 (20.6) |
| Staphylococcal infection final underlying cause of death | 216 (34.3) | 249 (32.3) | 344 (33.7) | 435 (34.2) | 483 (33.6) | 546 (34.3) | 2273 (33.8) |
| Total deaths with any mention of staphylococcal codes | 630 (100) | 772 (100) | 1020 (100) | 1271 (100) | 1439 (100) | 1591 (100) | 6723 (100) |
ICD-9=international classification of diseases, 9th revision.
MRSA=methicillin resistant Staphylococcus aureus.
Acknowledgments
We thank Alan Baker, Cleo Rooney, and Georgia Duckworth for all their help.
Footnotes
Funding: NSC was funded by the Office for National Statistics and MC was funded by the Public Health Laboratory Service. No additional funding was obtained.
Competing interests: None declared.
References
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