Principles of Screening
The ideal screening test must also be sensitive and specific. Table 1 illustrates these principles. The test must be able to correctly identify patients who have the disease (sensitivity) and those who do not have the disease (specificity). For example, for a medical test to determine if a person has a certain disease, the sensitivity to the disease is the probability that if the person has the disease, the test will be positive. The sensitivity is the ratio of true positive (TP) results to all diseased cases in the population. The specificity to the disease is the probability that if the person does not have the disease, the test will be negative. That is, the specificity is the ratio of true negative results to all negative cases in the population.
Prevalence of a disease in a statistical population is defined as the total number of cases of a given disease in a specified population at a specified time and/or the ratio of the number of cases of a disease present in a statistical population at a specified time and the number of persons in the population at that specified time. For example, in 2007, according to the U.S. Centers for Disease Control and Prevention (CDC), obesity prevalence was 33.3% among men and 35.3% among women.1,2
Incidence of a disease is defined as the number of new cases of the disease occurring in a population during a defined time interval. It is a measure of the risk of disease. In short, prevalence is a proportion, and incidence is a rate. Prevalence involves all affected persons, regardless of the date of contracting the disease. To illustrate, diabetes mellitus is becoming a major health issue in the United States. The number of existing cases (prevalent cases) and the number of new cases (incident cases) of diabetes are increasing, and most of this increase is not a result of the aging of the U.S. population. Between 1980 and 1996, the number of persons with diagnosed diabetes increased by 2.7 million. In 1996, about 8.5 million persons in the United States (3.2% of the population) reported that they had diabetes mellitus.3 The incidence of diabetes increased in the early 1980s but leveled off in the middle of the decade. It then increased in the 1990s. In 1996, the age-adjusted incidence of diabetes (2.79 per 1000 population) was 18% higher than the incidence in 1980 (2.36 per 1000 population). In the 1990s, the number of new cases of diabetes averaged more than 760,000 per year.3
PERIODIC HEALTH EXAMINATION
Since the 1980s, the American College of Physicians, the American Medical Association, the U.S. Preventive Services Task Force (USPSTF), and the U.S. Public Health Service have all agreed that routine annual checkups for healthy adults should be abandoned in favor of a more selective approach to preventing and detecting health problems. Nevertheless, the public expects to be given a comprehensive annual physical examination and extensive routine testing.4 Table 2 is the 2006 USPSTF Guide to Clinical Preventive Services.5 As Dr. Reinhart indicated in his letter to the editor, there are “probably very few of us, at any age, who would not be candidates for at least some health advice, such as on diet, exercise, or substance use or abuse. It also seems logical to believe that at least to some degree, repetition of health advice may have cumulative value.”6 Lastly, better patient adherence to a healthy lifestyle can lead to better health outcomes. A major factor promoting such adherence is the doctor-patient relationship or rapport with a physician. The annual physician examination is an extremely valuable building block in achieving such rapport.6
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