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| Journal of the New Zealand Medical Association,
17-December-2004, Vol 117 No 1207
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Under-reporting of diabetes on death certificates among a
population with diabetes in Otago Province, New Zealand
Kirsten Coppell, Karen McBride, Sheila Williams
Diabetes prevalence is increasing worldwide, and studies
have consistently shown excess death rates amongst diabetic populations compared
with the general population.1,2,3 However, the
impact of increasing diabetes prevalence on mortality rates is difficult to
determine. Death certificates are the only routine source of mortality
information with which to monitor the national burden of diabetes-related
deaths, but it has been repeatedly shown that diabetes is under-reported on
death certificates worldwide.1,4,5 Thus, the
contribution of diabetes to cause of death is usually
underestimated.6
The degree of under-reporting of diabetes on death
certificates varies between countries,1,5,7 as
does death certificate coding practices, although the same World Health
Organization (WHO) rules and guidelines are usually
used.8,9 This partly stems from differences in
opinion about the causal role of diabetes when it is associated with other
conditions such as cardiovascular disease.8,10
Also, coding practices have changed over time resulting in inconsistent time
trends. For example, in Australia, higher death rates attributable to diabetes
were reported in the 1940s compared with the 1950s and
1980s11—and in the Oxford region,
England, diabetes mortality rates based on underlying cause decreased stepwise
between the 1984-92 and 1993-99 study time
periods.12 Thus, the validity of comparisons of
diabetes mortality rates between countries and over time is questionable.
In New Zealand, there is only one published report
estimating the degree of under-reporting of diabetes on death
certificates.5 For this recent
Christchurch-based study, it was necessary to link several health information
sources to identify deceased diabetic patients. The Otago Diabetes Register,
established in 1998, enables diabetes-related data for enrolled individuals to
be directly linked with death certification information.
The purpose of this study was to use data from the Otago
Diabetes Register to estimate the degree of under-reporting of diabetes on death
certificates, and to describe the population of enrolled patients known to have
died during the 6-year period to 31 December 2003.
MethodsThe Otago Diabetes Register was
established as part of the Otago Diabetes Project to monitor and evaluate
diabetes care in the Otago region of New Zealand. The project also involved
establishing or updating general practice diabetes registers, organising
education sessions for general practitioners (GPs) and practice nurses and
developing and implementing guidelines for the management of core aspects of
diabetes care. Details of how general practice registers and the regional
diabetes register were established have been previously
described.13
Briefly, a project nurse established or updated general
practice diabetes registers for participating GPs (about 95% of all GPs in the
region). Identified diabetic patients were sent an invitation from their GP to
participate in the project, along with an explanatory pamphlet, a consent form
and a stamped addressed envelope for return of the form. Consent was also
obtained opportunistically, when patients attended their general practice or the
local retinal screening programme.
Data (including demographic details, type of diabetes
and year of diagnosis, dates and results of retinal and foot examinations,
diabetes complications, diabetes medication, and dates and results of
biochemistry tests) were collected annually from general practice medical
records. For patients who had attended Dunedin Hospital’s Outpatient
Diabetes Clinic or Eye Department, checks were made for missing data. If
patients’ diabetes type was uncertain, this was checked with the local
specialist diabetes clinic.
Those patients who had died during the previous 12
months were noted as such on the regional register. The vital status of the 279
patients who had moved from the Otago region or had changed to a
non-participating GP during the 6-year study period was not known. Between 1998
and 2003, the annual number of enrolled alive diabetic patients living in the
Otago region increased from 1693 to 3387, which was about 71% of the estimated
4800 people with diagnosed diabetes in the province.
Copies of death certificate information were obtained
from the national Births, Deaths and Marriages office, Department of Internal
Affairs for patients known to have died. ICD-10 was used to code all diseases
recorded on the death certificate.14 The date
of death, place of death, and causes of death were added to the Otago Diabetes
Register—a Microsoft Access-based program developed by the project.
Means and standard deviations, or frequencies and
percentages based on the patients’ last review, were calculated for
variables of interest. Student’s t -tests, Chi-squared tests, or
Fisher’s exact test (as appropriate) were used to examine differences
between men and women.
Ethical approval was obtained from the Southern
Regional Health Authority Ethics Committee.
ResultsAt 31 December 2003, 4320 diabetic
patients had ever been enrolled on the Otago Diabetes Register, of whom 509
(11.8%) were known to have died. Death certificate information was matched for
508 enrolees who had died, of whom 253 were females and 255 were males. Most
people (96.1%) were of European descent, with 3.0% self-identifying as Maori. Of
the 508 with matched death certificate information, 17 had type 1 diabetes, 482
had type 2 diabetes, and 9 had diabetes secondary to another condition or
steroid medication. More than one-half (57.1%) of the patients died in a
hospital. The proportion of males who died at their own home (22.4%) was higher
than for women (13.0%). Other places of death were rest homes (17.9%), hospices
(6.3%), and the community (1.0%).
For those with type 1 diabetes, 4 were females and 13 were
males. Five people, all males, were aged less than 50 years at death and died in
their own home. The immediate cause of death for three of these five males was
diabetic ketoacidosis and the other two died from a hypoglycaemic
event.
The characteristics of the 491 patients with type 2- or
secondary-diabetes are shown in Table 1. Overall, the mean duration of diagnosed
diabetes was about 12 years. Females were slightly older at both the time of
diagnosis of diabetes and time of death compared with males. About one-quarter
used insulin treatment prior to death, and a further one-quarter were being
treated with diet only. Over one-half were prescribed an ACE inhibitor during
the review year prior to death. A further 48 people (9.8%) had previously been
prescribed an ACE inhibitor, but were no longer taking it. (Reasons for stopping
their ACE inhibitor therapy were not recorded on the Otago Diabetes Register.) A
statistically significant higher proportion of females were prescribed
antihypertensive treatments (excluding ACE inhibitors) compared with males
(p=0.0002).
Diabetes was mentioned on the death certificates of 55.1% of
all 508 diabetic patients. For seven people, diabetes was listed as the
immediate cause of death. Diabetes was not recorded anywhere on the death
certificate of one person with type 1 diabetes. People who died in hospital or
their own home were more likely to have diabetes mentioned on their death
certificate, 59% and 62% respectively, compared with those who died at other
places: rest home (50%), hospice (31%), and community (40%).
Table 2 shows the clinical characteristics of those who had
diabetes mentioned on their death certificate compared with those who did not
have diabetes mentioned on their death certificate. Overall, the group with
diabetes mentioned on their death certificate were younger at the time of
diagnosis and duration of diabetes was about 5 years longer than those where
diabetes was not mentioned. A significantly higher proportion of the group who
did not have diabetes mentioned on their death certificate were using diet only
treatment for glycaemic control compared with the group who had diabetes
mentioned (p=0.0). The reverse was observed for insulin treatment (p=0.001).
Similar proportions of each group used oral hypoglycaemic treatments.
Table 3 shows the immediate causes of death by major disease
category as recorded on the death certificate. Almost 50% of deaths were due to
a circulatory system disorder, either cardiovascular or cerebrovascular disease.
Respiratory diseases and neoplasms were also frequent causes of death. A
neoplasm was the cause of death for a significantly higher proportion of the
group with no diabetes mentioned on the death certificate compared with the
group with diabetes on the death certificate.
DiscussionUnder-reporting of diabetes on death
certificates was recognised worldwide some time
ago,4 but this practice
continues.7 This study found that 45% of people
with documented diabetes had no mention of diabetes on their death certificate.
While this is consistent with a recent Christchurch
study,5 some studies have found that as many as
73% of diabetic patients have diabetes mentioned at any level on the death
certificate, but this was amongst an insulin treated diabetic
population,1 whereas other studies have found
as few as 36% of death certificates amongst a diabetic population mention
diabetes.7
The proper completion and accuracy of death certificates has
been questioned in several countries, including New
Zealand.15,16,17 Clinical diagnoses may be
erroneous, but often little can be done about incorrect or unknown diagnoses.
For known diseases, the sequence of events leading to death may be entered
incorrectly on the certificate or important events
omitted.9 Guidelines and calls to ensure that
doctors complete certificates correctly have been made in many
countries.16,17 In New Zealand, directions for
completing death certificates are included on the front cover of each book of
certificates, and detailed in a booklet published by
NZHIS.18 As diabetes is not always the
immediate, underlying, or contributing cause of death it will correctly not be
recorded on the death certificate, but this is unlikely to be the explanation
for the absence of diabetes on all 45% of the 508 death certificates that we
examined.
The range of diseases known or thought to be associated with
diabetes, including some cancers such as pancreatic, liver and more recently
bowel cancer,19,20 may not always be
recognised, and this may contribute to the under reporting of diabetes on death
certificates. There may be other explanations. Nevertheless, because of the
complex nature of diabetes, it has been suggested that diabetes should always be
recorded on the death certificate of all those with this condition, regardless
of whether it is considered to be the underlying or contributing cause of
death.4
The most recent published mortality data for New Zealand
shows that for the year 2000, 1455 people died in the Otago DHB
region.21 From this study, it is known that in
2000, at least 115 people with diabetes died (8% of the total number of deaths
in the Otago region). This proportion could be an underestimate, as not all
people with diabetes are enrolled on the Otago Diabetes Register and hence were
not included in this study, and generally up to 50% of people with diabetes have
not had the condition diagnosed.
The median age at death for this diabetic study population
(79.6 years for females and 78.2 years for males) compares favourably with the
median age at death for the general New Zealand population. For the 2000-2002
period, half of female
deaths occurred at ages 81 years and over, and half of male deaths occurred at
ages 75 years and over.22 A similar
observation was noted using data from the Skaraborg Diabetes Registry, Sweden,
where diabetic patients aged over 80 years had a survival similar to that of the
background population,23 as did a group of
Scottish men who were aged over 65 years at the time type 2 diabetes was
diagnosed.24 This supports the suggestion that
the onset of diabetes at an older age may not decrease life
expectancy.25 However, quality of life may be
reduced, particularly if diabetes related complications are present. At the
other end of the age spectrum, the deaths of 5 males with type 1 diabetes aged
less than 50 years from diabetic ketoacidosis or hypoglycaemia was surprising
and concerning.
The higher proportion of females prescribed antihypertensive
medication (compared with males) was an unexpected finding, but it is consistent
with results of the most recent New Zealand Health Survey which found that the
prevalence of self reported hypertension was higher amongst females than males
in the 65–74 and 75+ age groups.26 Also,
among patients aged over 60 years registered at Swedish primary health centres
males had generally better blood pressure control (defined as less than 140 mmHg
systolic and/or 85 mmHg diastolic) than
females.27
Cardiovascular disease was not unexpectedly the most common
cause of death. While this study did not compare cardiovascular mortality rates
between diabetic and general or non-diabetic populations, many studies have
found that the death rate for cardiovascular disease (particularly ischaemic
heart disease) is higher among diabetic populations compared with the general
population.1,3,28
There is no reason why this would not be the case for the Otago region.
Diabetes prevalence is increasing worldwide, yet the impact
on mortality cannot be accurately monitored. Under-reporting of diabetes on
death certificates was recognised more than two decades ago, and this study
found that this practice continues. If the impact of the diabetes epidemic on
mortality is to be monitored appropriately in New Zealand, attention needs to be
given to improving the completion of death certificates, including always
recording diabetes when it is present, irrespective of whether it is considered
to be the underlying or a contributing cause of death.
Author information:
Kirsten J Coppell, Senior Research Fellow, Edgar National Centre for
Diabetes Research, University of Otago, Dunedin; Karen McBride, Project Nurse
Co-ordinator, Otago Diabetes Project, Diabetes Otago Inc, Dunedin;
Sheila Williams, Biostatistician, Department of Preventive and Social
Medicine, University of Otago, Dunedin
Acknowledgements:
The Otago Diabetes Project was funded through contracts with the
following health funders: Southern Regional Health Authority, Health Funding
Authority, and Otago District Health Board. Additional funds were received from
Novo Nordisk Pharmaceuticals Ltd to undertake this study.
We acknowledge the invaluable assistance of Phillipa Mann
(Research Assistant), Claire Lamb (Project Nurse, Otago Diabetes Project), and
Jane Smith (Team Leader, Medical Records and Coding, Dunedin Hospital). We thank
all GPs, practice nurses, and diabetic patients who participated in the Otago
Diabetes Project.
Correspondence: Dr
Kirsten Coppell, Edgar National Centre for Diabetes Research, Department of
Medical and Surgical Sciences, University of Otago, PO Box 913, Dunedin. Fax:
(03) 474 7641. email: kirsten.coppell@stonebow.otago.ac.nz
References:
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