PhD thesis - University of Hertfordshire Research Archive

Methods.
The Health Protection Agency Centre for Infections maintains a database of
all certified deaths, reported to the Office for National Statistics, where an
infection was recorded as the certified underlying or contributory cause of
death. These represent approximately 10-15% of all deaths reported
annually. Descriptions of the underlying or contributory causes are provided,
in addition to their relevant International Classification of Diseases (ICD)
coding (ICD-9 from 1993-2000 and ICD-10 from 2001 to 2006).
Deaths with any link to campylobacters were identified by searching the
accompanying text fields for instances of „camp*‟. For deaths reported from
2001 to 2006 those attributed directly to campylobacters were identified
where the underlying cause field was coded as A045 („Campylobacter
enteritis‟). For deaths from 1993-2000 only the first four digits of the death
codes were available, and therefore campylobacter deaths (ICD-9 code
008.43) were coded as 008.4 („Intestinal infections due to other specified
bacteria‟). Accordingly, campylobacters were assigned as the underlying
cause for those deaths where the underlying cause was coded as 008.4 and
campylobacters were the only infectious agent recorded.
Data on all cases of campylobacteriosis reported to the Health Protection
Agency between 1993 and 2007 were extracted from the national laboratory
database to act as a denominator data for the calculation of case fatality
rates. Relative risks and 95% confidence intervals were calculated using
Stata version 10 (Stata Corporation, 1999) and chi squared tests calculated
using Epi Info (Dean et al., 1996).
To assess which factors might lead to the underestimation of campylobacter-
associated mortality, a binary outcome variable was created to compare
those deaths where campylobacter was not recorded as the underlying
cause versus those where it was. Explanatory variables were created to
represent ICD-9 or ICD-10 coding, gender, patients who were elderly or who
had a recorded underlying condition. An additional variable was created to
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