A primary goal of health care is to prevent disease or detect it early enough that intervention will be more effective. Strategies for disease screening and prevention are driven by evidence that testing and intervention are practical and effective. Currently, most screening tests are readily available and inexpensive. Examples include tests that are biochemical (e.g., cholesterol, glucose), physiologic (e.g., blood pressure, growth curves), or radiologic (e.g., mammogram, bone densitometry) or that involve tissue specimens (e.g., Pap smear). In the future, it is anticipated that genetic testing will play an increasingly important role in predicting disease risk (Chap. 63). However, such tests are not widely used except for individuals at risk for high-penetrancegenes on the basis of family or ethnic history (e.g., BRCA1, BRCA2). The identification of low-penetrance but high-frequency genes that cause common disorders such as diabetes, hypertension, and macular degeneration offers the possibility of new genetic tests. However, any new screening test, whether based on genetic or other methods, must be subjected to rigorous evaluation of its sensitivity, specificity, impact on disease, and cost-effectiveness. Physicians and patients are introduced continually to new screening tests, often in advance of complete evaluation. For example, the use of whole-body CT imaging has been advocated as a means to screen for a variety of disorders. Though it is appealing in concept, there is currently no evidence to justify this approach, which is associated with high cost and a substantial risk of false-positive results.
This chapter will review the basic principles of screening and prevention in the primary care setting. Recommendations for specific disorders such as cardiovascular disease, diabetes, and cancer are provided in the chapters dedicated to those topics.
Basic Principles of Screening
In general, screening is most effective when applied to relatively common disorders that carry a large disease burden (Table 4-1). The five leading causes of mortality in the United States are heart diseases, malignantneoplasms, accidents, cerebrovascular diseases, and chronic obstructive pulmonary disease. Thus, many prevention strategies are targeted at these conditions. From a global health perspective, these conditions are priorities, but malaria, malnutrition, AIDS, tuberculosis, and violence also carry a heavy disease burden (Chap. 2).
Table 4-1 Lifetime Cumulative Risk
| Save Table
Table 4-1 Lifetime Cumulative Risk
|Breast cancer for women||10%|
|Cancer of the cervix for womena||2%|
|Domestic violence for women||Up to 15%|
|Hip fracture for white women||16%|
A primary goal of screening is the early detection of a risk factor or disease at a stage at which it can be corrected or cured. For example, most cancers have a better prognosis when identified as premalignant lesions or when they are still resectable. Similarly, early identification of hypertension or hyperlipidemia allows therapeutic interventions that reduce the long-term risk of cardiovascular or cerebrovascular events. However, early detection does not necessarily influence survival. For example, in some studies of lung cancer screening, tumors are identified at an earlier stage but the overall mortality rate does not differ between screened and unscreened populations. The apparent improvement in 5-year survival rates can be attributed to the detection of smaller tumors rather than to a real change in clinical course after diagnosis. Similarly, the detection of prostate cancer may not lead to a difference in the mortality rate because the disease is often indolent and competing morbidities, such as coronary artery disease, may ultimately cause mortality (Chap. 82).
Disorders with a long latency period increase the potential gains associated with detection. For example, cancer of the cervix has a long latency between dysplasia and invasive carcinoma, providing an opportunity for detection by routine screening. It is hoped that the introduction of new papillomavirus vaccines will provide additional disease prevention, ultimately reducing reliance on screening for cervical cancer. For colon cancer, an adenomatous polyp progresses to invasive cancer over 4–12 years, providing an opportunity to detect early lesions by fecal occult blood testing (FOBT) or endoscopy. In contrast, breast cancer screening in premenopausal women is more challenging—and controversial—because of the relatively short interval between development of a localized breast cancer and metastasis to regional nodes (estimated to be −12 months).
Methods of Measuring Health Benefits
It is not practical to perform all possible screening procedures. For example, screening for laryngeal cancer in smokers is not currently recommended. It is necessary to examine the strength of evidence in favor of screening measures relative to the cost and risk of false-positive tests. For example, should ultrasound be used to screen for ovarian cancer in average-risk women? It is currently estimated that the unnecessary laparotomies triggered by finding benign ovarian masses would cause more harm than the benefit derived from detecting the occasional curable ovarian cancer.
A variety of endpoints are used to assess the potential gain from screening and prevention interventions:
The number of subjects screened to alter the outcome in one individual. It is estimated, for example, that 731 women ages 65–69 would need to be screened by dual-energy x-ray absorptiometry (DEXA) and then treated appropriately to prevent one hip fracture from osteoporosis.
The absolute and relative impact of screening on disease outcome. A meta-analysis of Swedish mammography trials (ages 40–70) found that −1.2 fewer women per thousand would die from breast cancer if they were screened over a 12-year period. By comparison, −3 lives per 1000 might be saved from colon cancer in a population (ages 50–75) screened with annual FOBT over a 13-year period. Based on this analysis, colon cancer screening may actually save more womens lives than does mammography. The impact of FOBT (8.8/1000 versus 5.9/1000) might be stated either as 3 lives per 1000 or as a 30% reduction in colon cancer death; ...
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