A system for the collection of data on objective measures of health, such as the NHANES operated by the National Center for Health Statistics, is an essential element of any national health data collection effort. Such surveys have been conducted as part of the overall data strategy of the Public Health Service since the 1960's, with examination surveys conducted at regular intervals from 1960 until the completion of the most recent survey in 1994.
Without objectives measures, collection on a representative population using standardized measures, any analysis of health status, health care utilization, expenditures, preventive strategies or performance monitoring will be severely compromised. Because of the costs and complexity of conducting nationally based examination surveys, it has been necessary for various agencies to consolidate their analytic needs. The NHANES survey is designed to obtain general health information needed by all agencies of the Department as well as the unique needs of each agency. Nearly all institutes of the NIH, FDA, EPA, CDC, and others have collaborated on the third NHANES which was recently completed.
Although the specific analytic objectives of data collected by examination surveys are extensive, they can be thought of as falling into two main classes.
• Information obtained from interviews or from medical records reflect how individuals view their own health and what they do about it in terms of seeking health care, the kind of care sought and their ability to perform normal functions. These mechanisms are not only a function of objective health status, but of social, environmental and behavioral circumstances. Examination studies provide information on objective indicators of health and disease, and therefore, provide the only source of information on undiagnosed conditions. This is particularly important when access and utilization of health care services, as well as knowledge of disease process, are not evenly distributed in the population. Without such information it is impossible to set research and intervention priorities, evaluate interventions, monitor performance and evaluate general health status. Objective measures of health also can be used to create predictive models of health status and the need for health care. Such measures are better predictors than are measures derived from other sources.
• Information from examination surveys provide standard distributions against which other data can be benchmarked. Most studies which rely on objective health measurement, such as clinical trails, are limited to select populations or geographic areas. It is essential that researchers be able to compare their populations to national norms in order to more fully interpret their findings. Nationally based examination surveys of sufficient size are the only means to develop these normative distributions. Information from national surveys is used to more efficiently plan intensive clinical or epidemiologic investigations, and also has clinical implications as evidenced by the use of growth charts to evaluate the development of children.
A few selected specific examples of the use of examination data collected by the NHANES is provided below.
Hepatitis viruses: NHANES is the only source of information on the number of people infected with the hepatitis viruses in the United States. Infection with these viruses is primarily asymptomatic (only 30 percent of infections have classic symptoms that would be diagnosed by a physician) but there are long term consequences with regards to chronic liver disease. Data from NHANES II (1976-1980) were the first evidence that hepatitis B was disproportionally infecting the minority population in the U.S. Until serologic tests were included in this survey, the epidemiology of this viral infection was estimated from blood donors, a very unrepresentative sample of the population. Based on NHANES II results, a universal vaccination program was implemented by CDC to eradicate hepatitis B transmission in the U.S. Future NHANES will help monitor the effectiveness of this program and further evaluate the epidemiology of all the hepatitis viruses on a national basis.
HIV: Measurement of prevalence of antibody to human immunodeficiency virus (HIV) on a representative sample of the U.S. population has provided CDC with a serologic basis for establishing an estimate of the number of people infected with this virus.
Tetanus: Tetanus antitoxin levels in the NHANES III population provided data for the first time to document that a substantial proportion of 10-16 year olds are not immune to tetanus. Although information obtained from interviews resulted in an estimate that 96 percent of children were vaccinated by the time they entered school, data from direct physical examinations revealed that these children had not received the required number of shots (5), and therefore were not fully protected. Only serologic data on a representative sample can provide this type of analysis of the immunization status of the population, to fully examine the medical practice and access to care in the U.S.
Herpes simplex type 2: Testing for antibody to Herpes simplex type 2 has been conducted in the previous two NHANES cycles as an index of sexually transmitted diseases (STD) in the population. These data linked to the STD risk behaviors provide evidence for an upward trend in STD that has helped redirect prevention efforts.
Helicobacter pylori: H. pylori has been confirmed as the causative agent of gastritic ulcers in adults and children. Recent evidence has suggested that infection with this organism is a critical precursor to the development of gastric cancer. This infection is primarily asymptomatic, therefore information from physician office visits or clinical trials do not provide the "big picture" on who is getting infected needed to establish effective screening and prevention programs. Testing of antibody to this organism had been performed on NHANES III sera and will be included in future surveys to provide a mechanism to estimate who is infected with this organism and help determine risk for infection.
Oral Health: Data derived from the NHANES have been critical for monitoring oral health status, risk factors for disease, access to preventive and treatment services and other variables among the general population and special subpopulations. Dental diseases have a major impact on the economic productivity of the Nation. In 1989, more than 164 million hours were missed from work and more than 51.6 million hours of school were lost because of dental treatment and problems.
Osteoporosis: Each year there are nearly 170,000 wrist fractures, 500,000 vertebral fractures, and 250,000 hip fractures in the U.S. population. It has been estimated that the annual cost of osteoporosis is about $10 billion. The magnitude of this problem is likely to increase dramatically over the next few decades as the population ages. Important pieces of data are not currently available about the changes in bone mass in the population. During NHANES III, dual energy x-ray absorptiometry of the hip was included in the examination of a subset of the examinees.
Osteoarthritis (x-rays): Arthritis and back problems (one of the musculoskeletal pain syndromes) are among the most frequently occurring chronic conditions affecting the U.S. population. They have a substantial impact on the quality of life of the individual, on the use of health care resources, and on the nation's economy. The most frequent form of arthritis, osteoarthritis (OA), has a formidable impact on the burden of disability and dependence among older Americans. Despite the public health impact, OA remains an enigmatic condition. There is no consensus as to the etiology, clinical features and natural history. Data from NHANES I have been useful in identifying obesity as a risk factor for symptomatic knee osteoarthritis. NHANES data were also valuable in identifying repetitive knee use in the work place as a risk factor.
Lung function (Spirometry): NIOSH must include in its lung function medical screening recommendations to OSHA and MSHA appropriate reference values. Until the completion of NHANES III, the most appropriate reference values recommended by NIOSH were based on less than 300 subjects. Although NHANES III provided critical information and a larger population base, reference values need to be continually updated as the population ages. The information generated is also used in medical care practice by physicians to directly assess the status of their patients. EPA is a major user in community studies.
Eye examination (Fundus photograph): The National Eye Institute has estimated that the cost of eye disease is in excess of $22 billion per year. However, no data exist on the current prevalence of visual acuity impairment and major causes of visual impairment in the population. In order to plan services and institute preventive programs, they have proposed the inclusion of an eye component in the next NHANES.
Cardiovascular disease (ECG): Cardiovascular disease is the leading cause of death and disability in the United States, and a primary cause of acute hospital bed days and physician visits. The assessment of cardiovascular disease-related risk factors has been a central component of the NHANES. Data describing rates of morbidity and disability related to cardiovascular disease have provided important information to researchers, health providers and policymakers from the public and private sectors of the health field. The population distribution of serum lipids and lipoproteins will provide reference data for evaluating progress in reducing serum cholesterol levels in the population through the National Cholesterol Education Program. Electrocardiograms will provide reference data for NHLBI studies of cardiovascular disease in older adults. High blood pressure control and reduction of serum cholesterol levels are among the priority areas specified in the 1990 Health Objectives for the Nation. NHANES III data will provide essential data describing the prevalence, determinants, and levels of public awareness of blood pressure and blood cholesterol and their relationship to cardiovascular disease morbidity and mortality.
Diabetes--Fundus photography : Diabetes mellitus is well documented to be a major public health problem in the United States. Fundus photography is included for adults 40 years of age and older for the purpose of diagnosis of retinal disease. Because retinopathy is one of the earliest complications of diabetes to appear, baseline fundus photographs will be used to document the natural history of diabetes and to quantify risk factors for the complications of diabetes. Of equal importance to NHANES III is the fact that photographs can be used to study the prevalence and natural history of macular degeneration.
Gallbladder disease: The results of the study of gallbladder disease in the NHANES III will effectively complement the clinical research also sponsored by the NIDDK. A better understanding of the prevalence of gallbladder disease and of the risk factors for this disease will result in greater opportunity for the primary prevention of gallbladder disease.
Allergy: The allergy data collected will be used by several agencies. The National Institute of Allergy and Infectious Diseases (NIAID) will use the information to further its knowledge about the distribution and determinants of allergies in the United States. The Health Resources and Services Administration (HRSA) will use the data to determine the demands allergies make on the health care delivery system. The FDA will use the data on the standardized allergens to assess reactivity in the general U.S. population. The NHLBI will use the information to increase its understanding of COPD. The history of asthma and skin test reactivity will be required to study COPD in NHANES III. A complete history of allergies and their effect on daily living is also required to describe child health accurately.
Food Fortification Policy: FDA, CDC, NIH and OASH are using the NHANES III nutrition biochemistries and dietary data to explore the safe levels of fortifying the food supply with folate. There are safety issues for older Americans with potential B12 deficiency and the detailed physiological data are necessary for changing the diet of the population with some confidence that no harm will be done. The public health issue is that folate levels may be related to neural tube defects and heart disease. The PHS group is currently active in examining this issue. Additional biochemistries are in progress that will enable the NHANES database to address the issue in sufficient detail. Historically, FDA has used the information to formulate or review current fortification and safety policies for iron and vitamin A in food products.
Exposure to toxic substances: NHANES information based on blood and urine specimens has produced comparative data for the study of toxic waste dumps, soldiers from Desert Storm, and exposure of the population to 15 to 20 pesticides and volatile toxic substances such as benzene, toluene, pentachlorophenol and others. Cadmium a highly toxic element has also been measured and exposure information will be produced. This type of information can be vital to administrators monitoring the potential exposures and risks as they are identified.