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| EPI6-3: Sources Of Exposure Data (Continued) |
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| EPI6-3: Sources Of Exposure Data (Continued) |
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Similar considerations apply to the collection of information on health status. Once again, it is important that the information obtained should be of comparable quality in the exposed and non-exposed populations. With this proviso, the specific methods used will differ according to the hypothesis and population under study, but the main options include use of routine records (mortality, incidence, hospital admission, health insurance, general practitioner, etc) and the mounting of a special morbidity survey (using clinical examinations, biological testing or questionnaires).
The measurement of health status will only be considered briefly here. The most important consideration is that the type of information required for epidemiological studies may be different from that which is required in clinical practice. As with exposure data, the key issue is that information should be of similar quality for the various groups being compared. For example, suppose that the bladder cancer incidence in a particular geographical area is being compared with national incidence rates; then it would be inappropriate to conduct a pathological review and reclassification of the cases of the cancer identified in the area, since such a reclassification had not been made for the national data and the information would not be comparable. Rather, the cancer cases in the area should be classified exactly as they had been classified in routine national cancer statistics. Thus, the emphasis should be on the comparability of information across the various groups being compared.
The types of health outcome data used in epidemiological studies include: mortality; disease registers; health service records; and morbidity surveys. These can be grouped into data based on routinely collected records, and morbidity data that is collected for a specific epidemiologic study.Most countries maintain comprehensive death registration systems at the national or regional levels, and cause of death information for identified deaths can be obtained by requesting copies of death certificates from national, state, or municipal vital statistics offices. In most instances the causes of death are coded by a nosologist trained in the rules specified in the International Classification of Diseases (ICD) volumes compiled by the World Health Organisation. Revisions to the ICD coding are made about every ten years, and in some instances the ICD code for a particular cause of death may change (Checkoway et al, 1989).
Some countries or states also maintain incidence registers for conditions such as cancer, congenital malformations or epilepsy. These have most commonly been established for cancer registration and the International Agency for Research on cancer (IARC) has been attempting to encourage the establishment of cancer registries and to standardise methods of cancer registration throughout the world (Jensen et al, 1991). Provided that registration is relatively complete, then cancer registrations can provide valuable additional health status information (and increase the number of identified cases) in a cohort study. Furthermore, cancer registries are invaluable for identifying newly diagnosed cases who can be interviewed (while they are still alive) for population-based case-control studies.
In some instances, de facto disease registers may be established through disease notification systems. In particular, many Western countries have notification systems for occupational diseases. For example, in the United Kingdom, the Surveillance of Work Related and Occupational Respiratory Disease (SWORD) project was established in 1989 as a national surveillance scheme for occupational respiratory disease (Meredith et al, 1991).
Other routinely collected records that can be used for determining health status in cohort studies, or identifying cases for case-control studies, include hospital admission records, health insurance claims, health maintenance organisation (HMO) records, and family doctor (general practitioner records).
Jones et al. (1997) performed a record linkage study of pre-natal and early life risk factors for childhood onset diabetes mellitus. They identified 160 boys and 155 girls born during 1965-1986 who had been admitted to hospital in Oxfordshire, England, with a diagnosis of diabetes during 1965-1987. For each cases, up to eight controls were chosen from records for live births in the same area, matched on sex, year of birth and hospital or place of birth. They then linked the hospital records for each child to all of that child's hospital records and to his or her mother's maternity record. There were no significant associations between subsequent diabetes and birthweight, gestational age, birthweight for gestational age, maternal age and parity. There were non-significantly increased risks with not breastfeeding (OR=1.33, 95% CI 0.76-2.34) and with diabetes recorded in the mother during pregnancy (OR=5.87, 95% CI 0.90-38.3), and a significantly raised risk with pre-eclampsia or eclampsia during pregnancy (OR=1.48, 95% CI 1.05-2.10). They hypothesized that pre-eclampsia may be the result of an immunogenetic incompatibility between mother and fetus, and that this early immunological disturbance may be related to the incidence of diabetes later in life.
In some circumstances, routine records may not be available for the health outcome under study, or may not be sufficiently complete or accurate or use in epidemiological studies. Although this could in theory apply to mortality records, more commonly this is an issue for non-fatal conditions, particularly chronic diseases such as respiratory disease and diabetes. I will therefore focus on these conditions here. Such morbidity surveys may involve clinical examinations (e.g. a clinical history and peak flow measurements for asthma), more invasive testing (e.g. blood tests for diabetes), questionnaires, or a combination of these methods.
To take the example of asthma, the essential feature of the condition (at least in clinical and epidemiological terms) is variable airflow obstruction which can be reversed by treatment or is self-limiting (Pearce et al, 1998). This poses several problems with the use of "diagnosed asthma" in asthma prevalence studies, since the diagnosis of "variable airflow obstruction" usually requires several medical consultations over an extended period. It is therefore not surprising that several studies have found the prevalence of physician-diagnosed asthma to be substantially lower than the prevalence of asthma symptoms. Such problems of differences in diagnostic practice could be minimised by using a standardised protocol for asthma diagnosis in prevalence studies. However, this is rarely a realistic option since it requires repeated contacts between the study participants and physicians, and this is not possible or affordable in large-scale epidemiological studies. Thus, most epidemiological studies must, by necessity, focus on factors which are related to, or symptomatic of, asthma but which can be readily assessed on a particular day. The main options in this regard are symptoms and physiological measurements. In particular, standardised symptoms questionnaires have been developed for use in adults (Burney et al, 1994) and children (Asher et al, 1995).
Dowse et al (1990) studied the prevalence if non-insulin dependent diabetes mellitus (NIDDM) in adults aged 25-74 years in Mauritius. A random sample of 5892 individuals was chosen and 5080 (83.4%) participated. They used a 75g oral glucose tolerance test with fasting and 2-h postload blood collection. Glucose tolerance was classified according to the World Health Organisation (WHO) criteria (World Health Organisation, 1985). The prevalence of NIDDM was similar in men (12.1%) and women (11.7%). Age and sex-standardised prevalence was similar in Hindu Indians (12.4%), Muslim Indians (13.3%), Creoles (10.4%) and Chinese (11.9%). The authors comments that the findings in Indians were similar to those in other studies of Indian migrant communities, but the findings in Creoles and Chinese were unexpected. "Potent environmental factors shared between ethnic groups in Mauritius may be responsible for the epidemic of glucose intolerance".
Health status can also be measured by more general morbidity and "quality of life" questionnaires. Perhaps the most widely used questionnaire has been the Medical Outcomes Study Short Form (SF-36) (Ware, 1993). This includes scales to measure physical functioning, role functioning, bodily pain, mental health, and general health perceptions. The SF-36 scales have been widely used in clinical research in a wide variety of populations to assess overall health status.
