LETTERS

Authors' response

Colin D Mathers^I,1; Doris Ma Fat^I; Mie Inoue^I, Chalapati Rao^II; Alan D Lopez^II

^IEvidence and Information for Policy, World Health Organization, 1211 Geneva 27, Switzerland
^IISchool of Population Health, University of Queensland, Brisbane, Australia

Editor – We welcome the interest and debate that our paper¹ has stimulated. Our two major aims were to promote interest in assessing and addressing quality issues in cause-of-death attribution and to facilitate better interpretation of such data. We comment here on the specific points raised by Johansson et al.

Construction of the quality measure

We used three quality categories only in the print version of the paper. The details provided in Table 2 of the paper (available from: http://www.who.int/bulletin) enable readers to decide whether or not data for some countries are close to the boundaries of these categories. Our analyses of data from the WHO mortality database show that patterns of causes of death from countries with >90% completeness are stable and allow good inferences to be drawn on the cause of death pattern in the total population. Thus level of incompleteness and per cent coded to ill-defined categories should not be simply added as a measure of "data loss" as suggested by Johansson et al.

Quality of certification versus quality of coding

We have only analysed the data available to WHO, which consist of ICD-coded deaths by age and sex. It is not possible to infer from these data whether certification or coding is responsible for excessive proportions of ill-defined causes. Good-quality coding practice should include procedures to query and correct as far as possible certificates that yield an ill-defined code for the underlying cause. We assume that countries with high proportions of ill-defined categories do not implement such verification procedures at the coding stage. We used the term "quality of coding" to cover both certification and coding, but agree that it would be more accurate to refer to "quality of certification and coding".

Selection of causes counted as ill-defined

In selecting broad groups of ill-defined causes, we were constrained by the fact that a number of countries still report data in much aggregated form. For example, sudden infant death syndrome (SIDS) is not reported separately if the country uses the ICD-10² condensed list 1. We thus examined the proportion of deaths assigned to the entire chapter for "symptoms, signs and ill-defined conditions". Similarly, some of the causes proposed by Johansson et al. as ill-defined could not be examined across all countries. This type of analysis is certainly feasible for countries reportip ing data using detailed ICD codes.

Although a more refined analysis would exclude SIDS, it represents<0.5% of ill-defined deaths in those countries where the proportion of ill-defined deaths is high. Exclusion of SIDS would make no real difference to the results we reported. Similarly, while ICD-10 code C97 may not represent an ill-defined code for some deaths, it represents a highly variable proportion of total ill-defined cancer deaths, ranging from ca 1% in Finland or Denmark, to 20 –30% in France, Germany, and Switzerland. This suggests that it may be overused in some countries. In any case, its exclusion from the analysis would make little difference to our results.

For many of the additional causes mentioned by Johansson et al. it is not easy to decide statistically what proportion should be treated as ill-defined codes rather than appropriate underlying causes of death. Such quality issues are probably better addressed through specific recoding studies at country level.

We did not retain atherosclerosis (I70.9) as an underlying cause of death as it is more important from a public health perspective to know the nature of the resulting disease. ICD-10 Modification Rule C (Linkage) specifically moves assignment away from atherosclerosis and hypertension to the disease manifestations, principally cardiac, renal or cerebrovascular. The overuse of atherosclerosis as an underlying diagnosis does indicate a departure from ICD coding rules, and it is thus appropriate to include generalized atherosclerosis among the ill-defined cardiovascular codes. It would probably also be appropriate to treat I10 (Unspecified (primary) hypertension) in the same way.

Events of ill-determined intent (Y10-34) represent ca 0.1% or less of deaths in countries with well functioning medico-forensic systems (e.g., 0.05% of deaths in Australia). This probably represents a lower limit of deaths where intent is not possible to determine. As this category has a median value of 0.5% and ranges up to 5% in some countries, high values are likely to indicate inadequate medico-forensic investigation. While it would be possible to estimate an irreducible minimum for this category and subtract it for all countries, this would make little difference to the analysis we presented, and we opted for a simple and readily understood indicator.

Comparisons between countries without age adjustment

Differences in the age distribution of deaths do not explain the variations in use of ill-defined categories that we reported. For example, around 6.7% of deaths in Sweden are coded to ill-defined cardiovascular codes. In Finland and Australia, where the age distributions of deaths are comparable, the corresponding proportion is 1.3% and 2.8%, respectively. Also ca five times as many deaths are coded to the "ill-defined causes" chapter of ICD in Sweden than in Finland or Australia.

As far as we know, our paper is only the second to assess the quality and availability of data on causes of death globally.³ We chose a set of simple indicators, and summarized them using three broad categories to highlight the large variations in completeness and quality of cause-of-death information across both middle- and high-income countries as well as the huge lack of mortality data for low-income countries.

We look forward to the publication of more detailed analyses of the quality of death registration data. A cursory examination of cross-country variations in the use of many cause-of-death codes suggests that problems of consistent and comparable measurement are far greater for many causes of death than our analysis has identified. For example, among the countries of continental Latin America, there is a more than 100-fold variation in death rates for Alzheimer disease and other dementias.

Finally, it should be noted that, by highlighting the overuse and inappropriate use of some ICD codes, we did not mean that all use of such codes should be avoided, only their overuse. The huge differences across countries in use of these codes points to the existence of poor certification and coding practices that need to be debated and addressed. WHO and its Collaborating Centres can play an important role in supporting countries to improve the quality and relevance of death certification and coding practices if data on population levels of disease and injury are to be truly useful for the purposes for which they are intended.

Competing interests: none declared.

References

1. Mathers CD, Ma Fat D, Inoue M, Rao C, Lopez A.D. Counting the dead and what they died from: an assessment of the global status of cause of death data. Bull World Health Organ 2005;83:171-7.

2. International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10). Geneva: World Health Organization; 1994.

3. Ruzicka LT, Lopez AD. The use of cause of death statistics for health situation assessment: national and international experiences. World Health Stat Q 1990; 43:249-58.

1 Correspondence to Dr Mathers (email: mathersc@who.int).