Chapter 004. Screening and Prevention of Disease (Kỳ 1) pptx

Chapter 004. Screening and
Prevention of Disease
(Kỳ 1)

Harrison's Internal Medicine > Chapter 4. Screening and Prevention of
Disease
Screening and Prevention of Disease: Introduction
A primary goal of health care is to prevent disease or to 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), radiologic (e.g., mammogram,
bone densitometry), or tissue specimens (e.g., Pap smear, fine-needle aspirations).
In the future, it is anticipated that genetic testing will play an increasingly
important role for predicting disease risk (Chap. 64). However, such tests are not
widely used except for individuals at risk for high-penetrance genes based on
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, or 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 continuously
introduced 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 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, or cancer, are provided in the chapters dedicated

to these 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, malignant neoplasms, accidents,
cerebrovascular diseases, and chronic obstructive pulmonary disease. Thus, many
prevention strategies are targeted at these conditions. From a global health
perspective, these same conditions are priorities, but malaria, malnutrition, AIDS,
tuberculosis, and violence carry a heavy disease burden (Chap. 2).
Table 4-1 Lifetime Cumulative Risk

Breast cancer for women 10%
Colon cancer 6%
Cancer of the cervix for women
a


2%
Domestic violence for women Up to 15%

Hip fracture for Caucasian women

16%

a
Assuming an unscreened population.
A primary goal of screening is the early detection of a risk factor or disease
at a stage when 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 overall mortality 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 a real
change in clinical course after diagnosis. Similarly, the detection of prostate
cancer may not lead to a mortality difference because the disease is often indolent
and competing morbidities, such as coronary artery disease, may ultimately cause
mortality (Chap. 78).
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 papilloma virus vaccines
will provide additional disease prevention, ultimately reducing the 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. On the other hand,
breast cancer screening in premenopausal women is more challenging because of
the relatively short interval between development of a localized breast cancer and
metastasis to regional nodes (estimated to be ~12 months).