Comparing Swedish hospital discharge records with death certificates: implications for mortality statistics

Lars Age Johanssona,b and Ragnar Westerlingb

a Statistics Sweden.
b Department of Public Health and Caring Sciences, Unit of Social Medicine, Uppsala University, Sweden.

Reprint requests to: Lars Age Johansson, BV/HS, Statistics Sweden, Box 24 300, SE-104 51 Stockholm, Sweden.


    Abstract
 Top
 Abstract
 Introduction
 Material and Methods
 Results
 Discussion
 References
 
Background The quality of mortality statistics is of crucial importance to epidemiological research. Traditional editing techniques used by statistical offices capture only obvious errors in death certification. In this study we match Swedish hospital discharge data to death certificates and discuss the implications for mortality statistics.

Methods Swedish death certificates for 1995 were linked to the national hospital discharge register. The resulting database comprised 69 818 individuals (75% of all deaths), 39 872 (43%) of whom died in hospital. The diagnostic statements were compared at Basic Tabulation List level.

Results The last main diagnosis and the underlying cause of death agreed in 46% of cases. Agreement decreased rapidly after discharge. For hospital deaths, the main diagnosis was reported on 83% of the certificates, but only on 46% of certificates for non-hospital deaths. Malignant neoplasms and other dramatic conditions showed the best agreement and were often reported as underlying causes. Conditions that might follow from some other disease were often reported as contributory causes, while symptomatic and some chronic conditions were often omitted. In 13% of cases, an ill-defined main condition was replaced by a more specific cause of death.

Conclusions There is no apparent reason to question the death certificate if the main diagnosis and underlying cause agree, or if the main diagnosis is a probable complication of the stated underlying cause. However, cases in which the main diagnosis cannot be considered a complication of the reported underlying cause should be investigated, and assessments made of the feasibility and cost-effectiveness of routinely linking hospital records to death certificates.

Keywords Cause of death, death certificates, main diagnosis, hospital records, quality control, medical record linkage

Accepted 27 October 1999


    Introduction
 Top
 Abstract
 Introduction
 Material and Methods
 Results
 Discussion
 References
 
Mortality data are a cornerstone of epidemiological research and health monitoring. Other data sources, such as health surveys, inpatient statistics and problem-specific medical registers, may quite often provide data better suited for some particular purpose. So far, however, mortality statistics are more broadly attainable at the international level. Also, since instructions for the compilation of mortality statistics have formed an integral part of the International Classification of Diseases for more than half a century, they are arguably better co-ordinated.1

A number of quality problems have been identified, ranging from diagnostic difficulties to national bias in data processing.2–11 Inaccuracy in death certification is a main concern,2 and producers of official mortality statistics often take great pains to monitor and correct death certificates, for example by means of manual and computerized verification. Traditional editing techniques are heavily biased, however, since they will only capture evident errors. Completed forms that look correct, but do not correspond to the actual facts of the case—a serious problem in any assembling of statistical data—will not be discovered.12,13

Occasionally, clusters or random samples of death certificates have been compared to their corresponding medical records.14,15 While this approach avoids the bias inherent in traditional editing, the method is unfortunately far too slow and expensive for routine use. The result is also an assessment of data quality rather than an improvement in routinely produced statistics.

In many countries decedents have records in other computerized registers. Several studies have used register linkage to assess the quality of cause-of-death data, or to arrive at better estimates.10,16–21 Goldacre linked cause-of-death data for the Oxford region to hospital discharge records, and showed that mortality statistics do not necessarily reflect the actual disease pattern among people who die.22 It has been suggested that similar data linkage could also be used to improve the quality of mortality statistics.16 In this study we match hospital discharge data to death certificates at the national level and discuss the implications of our findings for the routine production of mortality statistics.

The definitions of hospital discharge main diagnosis and underlying cause of death do not coincide. The main diagnosis, as defined in ICD-9, is ‘the main condition treated or investigated during the relevant episode’. The underlying cause of death, on the other hand, is ‘(a) the disease or injury which initiated the train of morbid events leading directly to death, or (b) the circumstances of the accident or violence which produced the fatal injury’.1 For the purpose of this study, we thus assumed that different definitions might account for many discrepancies between main diagnosis and underlying cause of death. But we also assumed that some discrepancies might still be hard to explain, and might deserve further analysis.


    Material and Methods
 Top
 Abstract
 Introduction
 Material and Methods
 Results
 Discussion
 References
 
We obtained cause-of-death data for 1995 from the national cause-of-death register, which comprises all deaths among Swedish residents, whether occurring in Sweden or abroad. The causes of death are coded centrally at Statistics Sweden according to the international (English) version of the Ninth Revision of the International Classification of Diseases (ICD-9).1 The 1995 record format permitted a maximum of 33 registered conditions per person, in addition to the underlying cause of death. The total number of deaths in 1995 was 93 910 and Statistics Sweden received death certificates for 99.7% of these.

The Swedish Hospital Discharge Register is a compilation of each individual hospital's discharge records, performed first at regional level by the 26 health administrative authorities, who forward them to the National Board of Health and Welfare. According to the Board's estimates, about 99% of all public hospitalizations are included.23 Thus, the vast majority of inpatient deaths are covered, since Sweden has very few providers of private inpatient care.24 Discharges from nursing homes are not reported. While the Board of Health performs extensive checks on the data received, coding and data entry are performed locally at each hospital. The diagnoses are generally coded by the physician responsible for the discharge. In 1995, Swedish hospitals used the KS87, an adaptation of ICD-9.25 The Hospital Discharge Register has one record for each episode of hospital care, each record containing a unique patient identifier, particulars of the hospital department and the hospitalization, as well as a main diagnosis, a maximum of five contributing conditions, the external cause in cases of injury, and codes for any surgery performed.

Matching the personal identification numbers, we extracted data on all hospital discharges within one year prior to death. The resulting database comprised 224 794 hospital discharges (69 818 individuals), corresponding to almost 75% of all deaths. Of these, 39 872 people (43% of all deaths) died in hospital.

At detail level, the Swedish KS87 classification does not always coincide with the international English version of the ICD. The discrepancies, which are most apparent in the E series (external causes), would have caused spurious differences at both four- and three-character level. To avoid this, we decided to translate all condition codes in both registers into categories of the ICD-9 ‘Basic Tabulation List’ (BTL). The three-digit BTL is a cause-of-death shortlist of about 230 categories. It corresponds fairly well to the level of detail at which mortality statistics are generally analysed. Practical tests on compatible parts of the two classifications also indicated that BTL groups would not yield a dramatically different result from three-character ICD categories (data not shown). The BTL groups do not, however, cover all causes of death and can thus not be used for calculation of totals. For this reason, we used Statistics Sweden's extended version of the BTL, in which residual groups have been added throughout the list.26

We then compared the hospital discharge data with the conditions mentioned on the death certificate, concentrating on the main diagnosis of the last hospital discharge and the underlying cause of death.

To assess the general correlation between main diagnosis and underlying cause of death, we analysed how the correlation varied with sex, age, time elapsed from hospital discharge to death, main diagnosis, and the number of different conditions in the last hospital discharge record. We scanned the death certificates for any mention of the main diagnosis as either underlying or contributory cause. Since we believed that later developments might make hospital discharge data less useful if the patient had died some time after discharge, we decided to limit the remaining analyses to hospital deaths. For those, we examined the death certificate to see if it contained more specific diagnostic information than the hospital discharge record. We also tabulated the underlying cause for deaths in which the underlying cause did not match the main diagnosis, and checked if the agreement between main diagnosis and underlying cause varied with the autopsy rate.


    Results
 Top
 Abstract
 Introduction
 Material and Methods
 Results
 Discussion
 References
 
The last main diagnosis and the underlying cause of death agreed in 46% of cases (46% for males, 45% for females). The agreement varied somewhat with sex and age, with the best correlation for middle-aged women (Table 1Go).


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Table 1 Agreement on Basic Tabulation List level between last reported main discharge diagnosis and underlying cause of death, by age and sex. All deaths within a year from discharge, Sweden, 1995. N = 69 818
 
There was a rapid decrease in correlation between main diagnosis and underlying cause after discharge (Figure 1Go). While the correlation is almost 60% for hospital deaths, the level plummets once the patient has been discharged, and is below 40% for deaths within the first week after discharge. For all non-hospital deaths the correlation is only 28%.



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Figure 1 Agreement last reported main hospital condition—underlying cause by time elapsed from hospital discharge to death

 
The number of conditions reported in the last hospital record also influenced the degree of correlation, with generally greater agreement the fewer the number of reported conditions (Figure 2Go). If only one condition was reported, it agreed with the underlying cause in 59% of all deaths, as opposed to only 31% when seven conditions were reported on the last hospital discharge.



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Figure 2 Agreement last reported main condition—underlying cause by number of reported conditions in the Hospital Discharge Register

 
For hospital deaths, the main diagnosis had been reported somewhere on 83% of the certificates (as underlying cause in 59% of cases, as contributory cause in 24%) (Figure 3aGo). In contrast, certificates for deaths after discharge mentioned the last main diagnosis in only 46% of cases (underlying cause in 28%, contributory cause in 18%) (Figure 3bGo).



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Figure 3a Mention of main hospital condition on the death certificate. Hospital deaths. Basic Tabulation List level, Sweden, 1995. N = 39 872

 


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Figure 3b Mention of main hospital condition on the death certificate. Deaths outside hospital. Basic Tabulation List level, Sweden, 1995. N = 29 946

 
The main diagnosis itself had a dramatic impact on the correlation between main diagnosis and underlying cause (Table 2Go). Conditions in ICD-9 Chapter II (Neoplasms) showed the greatest correlation for both deaths at hospital and deaths after discharge (78% and 61% respectively), while those in Chapter XVI (Symptoms, Signs and Ill-defined Conditions) showed least correlation (42%, 3%).


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Table 2 Agreement on Basic Tabulation List level between last reported main discharge diagnosis and underlying cause of death, by main diagnosis. All deaths within a year from discharge, Sweden, 1995. N = 69 818
 
If the main diagnosis is an acute or otherwise dramatic condition, then it is also often reported as the underlying cause of death. Thus, malignant neoplasms generally showed the best correlation between main condition and underlying cause. Conditions like acute myocardial infarction, aortic aneurysm (part of BTL group 302), traffic accidents, and subarachnoid haemorrhage ranked almost as high. Table 3aGo lists the ten conditions with the highest degree of correlation among hospital deaths, for both malignant neoplasms and non-malignant conditions.


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Table 3a Examples of main discharge diagnoses often reported as underlying cause of death. Hospital deaths, Basic Tabulation List level, Sweden, 1995. N = 39 872
 
The 15 main diagnoses most frequently reported as contributory causes of death are shown in Table 3bGo. Only conditions with at least 30 occurrences are included. Main diagnoses of ‘secondary’ nature, like complications of medical care, circulatory insufficiency (part of BTL group 309), heart failure (part of BTL group 289), and nephrotic syndrome, were frequently reported on the death certificate, but not as the underlying cause. However, chronic conditions like diseases of the nervous system and pneumoconiosis were also often mentioned as contributory causes only, even if they had been reported as the main diagnosis.


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Table 3b Examples of main discharge diagnoses most often reported as contributory cause of death. Hospital deaths, Basic Tabulation List, Sweden, 1995. N = 39 872
 
Some main diagnoses were quite often omitted from the death certificates. This was often the case for symptoms and mental disorders. However, some chronic disorders quite likely to have contributed to death were also often omitted, e.g. chronic obstructive lung disease. The 15 main diagnoses most frequently not mentioned on the certificate, either as underlying or as contributory causes, are given in Table 3cGo.


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Table 3c Examples of main discharge diagnoses often not mentioned on the death certificate. Hospital deaths, Basic Tabulation List, Sweden, 1995. N = 39 872
 
An ill-defined main diagnosis is sometimes replaced by a more specific cause of death. Thus, for 30% of the cases with a main diagnosis in ICD-9 Chapter XVI (Symptoms, Signs and Ill-defined Conditions, BTL 460–469), the underlying cause was coded to another ICD chapter. For ill-defined conditions outside Chapter XVI, however, a more specific underlying cause of death in the same broad category had been used in only 9% of cases. As can be seen in Table 4aGo, this occurred most often for ill-defined cerebrovascular diseases, ill-defined respiratory conditions, and ischaemic heart disease.


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Table 4a Ill-defined main discharge diagnosis replaced by a more specific cause of death in the same two-character Basic Tabulation List group. Hospital deaths, Sweden, 1995. N = 39 872
 
If the main diagnosis and the underlying cause do not concur, the cause of death actually reported tends to be either a circulatory or a respiratory disease, e.g. ischaemic heart disease or pneumonia. Table 4bGo shows the 15 most common underlying causes of death for such cases.


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Table 4b Distribution of reported underlying cause of death for cases in which the main discharge diagnosis did not agree with the underlying cause, the 15 most common causes. Hospital deaths, Basic Tabulation List, Sweden, 1995. N = 39 872
 
Autopsy findings did not seem to influence the correlation to any very great extent. The autopsy rate was somewhat, but not substantially, higher for cases in which the main diagnosis did not concur with the underlying cause of death (Table 5Go).


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Table 5 Autopsy rate and agreement between main discharge diagnosis and underlying cause of death. Hospital deaths, Basic Tabulation List level, Sweden, 1995. N = 39 872
 

    Discussion
 Top
 Abstract
 Introduction
 Material and Methods
 Results
 Discussion
 References
 
In general, our results are not difficult to interpret. The slight variations in correlation between main diagnosis and underlying cause by age and sex probably reflect differences in disease pattern (Table 1Go). The decrease in correlation after discharge from hospital comes as no surprise—new conditions can develop, and conditions that did not seem to merit a mention at the time of discharge may subsequently take a serious turn (Figure 1Go). Further, the same physician will probably be responsible for both the hospital discharge diagnosis and for the death certificate if the patient dies at a hospital. If the patient dies after discharge, however, the death certificate will be issued by a primary care physician who might view the case differently than did the physician attending the patient at hospital. This might explain the sudden decrease soon after discharge. Unfortunately, we cannot evaluate this hypothesis since neither register identifies the physician.

We found the best correlation for the malignant neoplasms (Table 2Go), and it is not hard too see why. A malignant neoplasm, reported as the last main diagnosis, is obviously very likely to be the underlying cause of death. It also seems reasonable that a greater number of reported conditions in the discharge record correspond to a lower degree of correlation with the underlying cause (Figure 2Go). The more conditions present, the more the physician has to choose from when filling in the death certificate.

Like Goldacre,22 we distinguished three broad groups of main diagnoses when analysing hospital deaths; those likely to be reported as the underlying cause of death (Table 3aGo), those likely to be reported as a contributory cause of death (Table 3bGo), and those likely not to be mentioned on the death certificate (Table 3cGo). As to improving the quality of the death certificates, the first group is probably of least significance. The fact that the main diagnosis and the underlying cause agree does obviously not guarantee that the death certificate is correct. However, the likelihood of obtaining a more probable cause of death from consulting the hospital records seems small. For an example from our data see Table 6Go, relationship a.


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Table 6 Examples of different relationships between main discharge diagnosis and underlying cause of death and implications for the plausibility of the death certificate
 
The two other groups: i) where the main diagnosis is a contributory cause, and ii) where it is not reported at all, deserve a closer examination. Sometimes a difference between main diagnosis and underlying cause can be easily explained. If the last main diagnosis is a probable complication of the reported underlying cause, then the apparent discrepancy is logically consistent with the definitions of main diagnosis and underlying cause (Table 6Go, b). This applies whether the main diagnosis is reported on the death certificate or not. Indeed, the WHO instructions for completing the death certificate tell the physician not to report symptoms, but the condition that caused them27(Table 6Go, c). Therefore, if the main diagnosis is reported as a contributory cause of death, or even not reported at all, there is no reason to distrust the death certificate—provided that the main diagnosis is secondary to the underlying cause.

In other cases, however, the main diagnosis is not a probable complication of the stated underlying cause (Table 6Go, d). That does not, of course, necessarily imply that the certificate is wrong. The certifying physician may have had access to information that is not present in the hospital records, and the later of the two records could simply reflect a more considered opinion than the first. One should also allow for competing causes of death—if the patient suffered from several serious conditions, the physician may have had good reason to select one of them for the discharge register, and another for the death certificate. In addition, the hospital discharge register is in no way exempt from errors. In a Swedish study in which reported hospital discharge diagnoses were checked against medical records, the error rate was estimated to be 14% at the ICD four-digit level.28

Nevertheless, incompatible morbidity and mortality statements might indicate that something is wrong. Goldacre22 found that the underlying causes actually reported in non-concordant cases tend to be circulatory or respiratory conditions: ischaemic heart disease, acute myocardial infarction, heart failure, acute cerebrovascular disease and pneumonia. None of these conditions would arouse the curiosity of a statistical office, at least not when stated as cause of death for elderly people. By comparing death certificates with their corresponding hospital records, however, we might find an incompatibility that indicates a potential quality problem that should be investigated.

Obviously, a statistical edit based on the relationship between main diagnosis and underlying cause would not solve all quality problems. Non-hospital deaths, for example, require a different approach. Linkage will not help either if the main diagnosis and underlying cause are identical but both incorrect. Moreover, the cost and time required for the procedure might prove prohibitive, or the quality gains insignificant in relation to the effort.

Thus, a cost-benefit analysis is needed, including an analysis of which errors a method based on record linkage is likely to identify and which it is likely to miss. To that end we need a detailed, computerized algorithm that defines valid and invalid relationships between main diagnosis and underlying cause. Such an algorithm might be based on the ‘decision tables' (tables specifying relationships between conditions and used for the selection of the underlying cause of death), developed by the US National Center for Health Statistics.29 With this done, the error pattern for both types of certificates—valid and invalid relationships—could be investigated, for example by means of checking the death certificates against the medical records. The possible benefits for the mortality statistics could then be estimated.

Diagnostic information concerning hospital deaths is currently reported both to the local hospital discharge register and, via the death certificate, to the national cause-of-death register. It might be tempting to relieve physicians of some of their administrative burden by replacing the customary certification of death by an extract from the hospital discharge record. However, this study illustrates that in spite of the obvious relationship between the two registers they remain essentially different: the death certificate gives a case history, while the hospital discharge record provides a snapshot. Hospital discharge records and death certificates supplement each other, in that some conditions likely to be absent from one of the registers will presumably be found in the other. Suppressing either of the two sources of information would cause severe disruption of statistical trends and a serious impoverishment of basic epidemiological data.


    Acknowledgments
 
This study was made possible by a grant from the Vårdal foundation (project number V97 393), and by the support from the National Board of Health and Welfare and from Statistics Sweden.


    References
 Top
 Abstract
 Introduction
 Material and Methods
 Results
 Discussion
 References
 
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