Prevalence of Serious Psychiatric Morbidity in Attenders and Nonattenders to a Health Survey of a General Population

The Tromsø Health Study

Vidje Hansen, Bjarne K. Jacobsen and Egil Arnesen

From the Institute of Community Medicine, University of Tromsø, Tromsø, Norway.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The objective was to study the effect of serious psychiatric disorders on participation in a general health population study. This was done by linking the records of the Second Tromsø Health Study to the case register of a mental hospital. The participants in the Second Tromsø Health Study were 21,441 persons, the total population of men aged 20–54 and women aged 20–49 years who resided in Tromsø, Norway, in 1979. The authors found that both men and women with psychiatric illness had approximately 20% lower attendance rates. Nonattenders to the survey had 2.5 times higher prevalence of psychiatric disorders than did attenders of both sexes. Age, marital status, and various psychiatric diagnoses were all significant predictors of nonattendance. Nonattendance led to underestimation of the prevalence of psychiatric disorders in the population. The conclusion is that in general health studies, even those with high attendance rates, the estimates of prevalence of psychiatric disorders in the population are seriously affected by nonattendance. Prevalence ratios between groups of the population were not much affected by nonattendance.

epidemiologic methods; health surveys; morbidity; psychiatry

Abbreviations: ICD-9, International Classification of Diseases, Ninth Revision; PRR, prevalence rate ratio


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Population screenings have frequently been used to obtain information about the prevalence of diseases (including case finding) and the occurrence of risk factors for a number of diseases. Furthermore, these data are often included in cross-sectional analyses examining relations between risk factors for diseases and prevalent cases or between risk factors. Both when determining prevalence numbers and when determining cross-sectional associations, to be able to generalize, it is important to obtain information from a representative sample of the population. Previous studies indicate that nonattenders tend to be young, male, and single; to have a low socioeconomic status; to be smokers; and to have alcohol problems (1GoGoGoGoGoGoGoGoGoGoGoGo–13Go). To our knowledge, no study has tried to examine mental diseases in nonattenders, even if it is likely that such problems would be more common among them. In this study, we linked the data from a cross-sectional health survey of the general population to the records of admissions to a mental hospital. This made it possible to identify those invited persons who had formerly suffered from mental distress serious enough that they were admitted to a mental hospital. The focus of this paper will be on the pattern of differences between persons with psychiatric disorders who responded to the survey and those who did not, with regard to age, sex, psychiatric diagnosis, and marital status.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In 1979–1980, all men aged 20–54 and women aged 20–49 years, respectively, who resided in the municipality of Tromsø, Norway, were invited to participate in a general health survey with emphasis on risk factors for coronary heart disease. In 1980, Tromsø had a population of about 45,000 people. Located there is the only mental hospital serving the two northernmost counties of Norway.

The population invited to the Second Tromsø Health Study was composed of all men in the municipality born between 1925 and 1959 and all women born between 1930 and 1959. A total of 16,621 persons (78 percent) attended the physical examination. Further details of the methods of this survey have been given elsewhere (1Go). To identify those among the 21,441 persons in the study population who had been admitted to a mental hospital at least once in their lifetime, we used four sources: 1) the case files, which contained one file for every patient; 2) a protocol in which all admissions had been entered since the hospital was opened in 1961; 3) a case file containing one card for every patient who had been admitted since a psychiatric department was opened at the local general hospital in 1950; and 4) a computerized register of all admissions since 1978. These sources were checked against the file of the health study to identify those of the 21,441 persons who had been admitted prior to the survey invitation. Attendance rates of persons with and those without a prevalent psychiatric disorder and prevalences of psychiatric disorders in attenders and nonattenders were age adjusted by the direct method, using the age distribution in the total invited population as the reference population. The prevalent cases were divided into five diagnostic groups according to the psychiatric diagnosis at first admission. The classification used at the hospital during this period was different versions of the International Classification of Diseases, arrived at by consensual clinical evaluation. For the analyses in this paper, the diagnoses were recoded according to the Ninth Revision (ICD-9) (14Go). The diagnostic groups used here were schizophrenia (ICD-9 codes 295.0–295.9), other psychosis (ICD-9 codes 290.0–290.9, 293.0–294.9, and 296.0–299.9), substance abuse (ICD-9 codes 291.0–291.2 and 303.0–305.9), nonpsychotic depression (ICD-9 codes 300.4, 301.1, 309.0, and 309.1), and the rest, categorized as "other disorders." Information about marital status was obtained from the Central Register of Persons of Norway, along with the personal identification and address of each inhabitant of Tromsø in the relevant age groups. We could not find the marital status for only 112 persons, who lived in Tromsø but attended without being invited. Logistic regression analyses were performed with the statistical package SAS (15Go). The linkage between the registers received approval from the Norwegian Data Inspectorate.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The linkage identified 596 persons who had been admitted to a mental hospital before the survey. Of these, 368 (62 percent) responded to the survey. Table 1 gives the age distribution of the invited population, the crude and age-adjusted attendance rates for cases and noncases by gender, and prevalence rates of psychiatric disorders in attenders and nonattenders. Attendance rates were approximately 20 percent lower for psychiatric cases than for noncases. In the total group, the age-adjusted prevalence of psychiatric disorders was higher in men than in women (prevalence rate ratio (PRR) = 1.3, 95 percent confidence interval:1.1, 1.5), but when it was stratified for attendance, there were no significant differences between the sexes. For both men and women, however, the age-adjusted prevalence of psychiatric disorders in nonattenders was more than twice that of attenders (men: PRR = 2.4, 95 percent confidence interval: 2.0, 2.9; women: PRR = 2.6, 95 percent confidence interval: 2.0, 3.5). As summarized in table 2, whether one uses the prevalences in attenders or in the total invited population is of minor importance for the prevalence ratios comparing men and women and marital statuses.


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TABLE 1. Attendance rates (%) among psychiatric cases and noncases in the Tromsø Health Study, 1979–1980, and prevalence of psychiatric disorder in attenders and nonattenders

 

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TABLE 2. Prevalence rate ratios based on all subjects in the population and on attenders only, the Tromsø Health Study, 1979–1980

 
The results of the logistic regression analysis, with age, marital status, and psychiatric diagnosis as predictors of nonattendance, are shown in table 3. In both sexes, odds ratios for nonattendance decreased with age and were highest for the previously married subjects. For psychiatric diagnosis, the pattern depended on gender, with substance abuse having the highest odds ratio of nonattendance in men and schizophrenia having the highest odds ratio in women.


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TABLE 3. Predictors of nonattendance to a general health survey, the Tromsø Health Study, 1979–1980

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Epidemiologic research in Norway is facilitated by the unique person-number assigned to all inhabitants and the sectorized organization of the service system. This assures that persons registered as inhabitants of one municipality are always admitted to the mental hospital in that sector or transferred quickly if admitted elsewhere. Thus, the only persons whom we might have misclassified with respect to prevalent psychiatric disorders are those who were admitted prior to 1950, either in Tromsø or elsewhere in Norway and were not readmitted. Since the oldest persons in the Second Tromsø Health Study was born in 1925, it is not likely that many of the study population were admitted before 1950. Nevertheless, we might have lost some cases due to this, and thus, we cannot discount the possibility of a slight underestimation of the population prevalence in the older age groups.

In terms of the prevalence of having been admitted to a mental hospital, the observed level of 2–3 percent is somewhat lower than that in other studies conducted in the predeinstitutionalization era (16GoGo–18Go).

The reported differences in age, gender, and marital status between attenders and nonattenders are in accordance with earlier studies (1Go, 3GoGoGoGoGoGoGoGo–11Go). We have found that nonattenders to a general health survey have more than twice the risk of attenders of having a prevalent serious psychiatric disorder. The psychiatric disorders studied here are a mixed group of diagnostic entities, but all of them are rather serious, since admission to a mental hospital was warranted. Three Swedish studies (3Go, 4Go, 12Go) have reported that persons with alcohol problems are particularly likely to avoid health screenings. This is confirmed in our study, since the group of substance abusers consists almost exclusively of persons with alcohol problems. We have not identified any studies that report on attendance and psychiatric disorders other than alcohol problems. Some prospective studies have reported on the mortality of nonattenders by using death registers (3Go, 4Go, 9Go, 19Go). All have found that violent death and suicide are far more common in nonattenders than in attenders. Since persons who later commit suicide are likely to have had more mental distress at the time of the initial invitation to the health examination, we believe that these findings support our own in identifying more mental distress among nonattenders.

The Second Tromsø Health Study had high attendance rates (78 percent), ranging from 57 percent in men aged 20–24 years to 85 percent in men aged 50–54 and from 69 percent in women aged 20–24 years to 92 percent in women aged 45–49 (1Go). This makes the study sample quite representative of the total population within this age span, especially in men older than 29 years. The result of selective nonattendance by persons with psychiatric disorders is their underestimation. However, in a study with such high attendance rates as the Second Tromsø Study, differences in prevalence between groups in the study sample, i.e., differences in the prevalence of mental disorder in men and women, between diagnostic groups, and between marital statuses is shown here to be quite robust for the effect of nonattendance. Studies with lower attendance rates may, however, run into problems in this respect.

By contrast, the impact of selective nonattendance is more disturbing for case finding, as one loses 32–43 percent of the persons with psychiatric disorders when doing a general health study. This has also implications for somatic morbidity because people with serious psychiatric disorders have much higher somatic morbidity rates than does the general population (20Go).


    ACKNOWLEDGMENTS
 
The Second Tromsø Health Study was performed in cooperation with the National Health Screening Service, Oslo, Norway.


    NOTES
 
Reprint requests to Dr. Vidje Hansen, Institute of Community Medicine, University of Tromsø, N-9037 Tromsø, Norway (e-mail: vidje.hansen{at}ism.uit.no).


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication December 4, 2000. Accepted for publication July 9, 2001.