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The concept of preventive health care screening has been developed over the last 30 years through clinical trials of screening maneuvers as well as a theoretical statistical literature. Screening is used in general medical practice in two distinct ways: we screen for early disease, and we screen for risk factors for disease or injury (such as smoking, hyperlipidemia, or non-use of seatbelts). Both are important, despite the emphasis in what follows on screening.
When we make decisions about preventive health care screening, we consider both disease and test. Screening is most effective if limited to diseases that meet specific criteria and for which appropriate tests are available. Four criteria are required to make a target disease appropriate for screening:
1) The disease must have a detectable preclinical period, before the disease becomes clinically apparent and during which the disease can be detected by the screening test. Cervical cancer is a disease that can be clinically silent for years, yet diagnosed by a screening test.
2) The detectable preclinical period must be before the disease escapes from cure. Cancer of the pancreas, for example, is generally incurable regardless of when it is detected and so does not meet this criterion.
3) There must be treatment that is more effective if given earlier (at the time of screen detection) than later (at the time the disease becomes clinically apparent). Radical prostatectomy for early prostate cancer has not been proven to improve survival, for example, which is why routine screening for prostate cancer is extremely controversial.
4) The disease must be sufficiently common in the population, since the prevalence of a disease, together with the sensitivity and specificity of the test, determine the positive and negative predictive values of the test.
For a screening test to be effective, it must be sensitive, specific, have high predictive value, be feasible for broad use, and acceptable in terms of cost, risk and patient inconvenience. Clinicians should be aware, for example, that colonoscopy carries a 0.1 percent chance of perforation, that prostate biopsy has a 2 percent risk of bacteremia and that 95 percent of women report that mammography is uncomfortable.
The current standard for evaluating screening maneuvers is the randomized clinical trial. Most such studies have addressed cancer screening tests. The endpoint on which interpretation of these studies rests is reduction in cause-specific mortality. The reason for this is that screening tests may detect a cancer at an earlier stage even in the absence of more effective early treatment, so that people with screen-detected cancers will live longer and have a lower case-fatality rate at any time interval. This phenomenon is termed lead time bias. In addition, screening maneuvers detect a mix of fast-growing aggressive tumors that respond poorly to treatment and slow-growing tumors that do well regardless of treatment. The screen-detected mix overrepresents.
The Guide to Clinical Preventive Services
In 1989, the U.S. Preventive Services Task Force, commissioned by the U.S. Department of Health and Human Services, published the results of its survey of strategies for the prevention of disease. The Guide to Clinical Preventive Services was updated in 2006. The survey examines interventions involving 80 conditions and is available in print and on the Internet. While some of these assessments involve counseling, immunization and chemoprophylaxis, the majority of the report deals with screening. The survey does not analyze the screening procedures themselves.
Table 1: Screening tests recommended for all adults
• Diet history
• Nonfasting total cholesterol (men 35-64, women 45-64
• Physical activity history
• Papanicolaou smear (women 18-65)
• Tobacco, alcohol, drug use
• Mammogram +/- CBE (women 50-69)
• Sexual history
• Colon cancer screening (patients over 50)
• Blood pressure
• Rubella serology or vaccination history
• Height and weight (fertile women of childbearing age)
Table 2: High risk groups (recommended screening tests in parentheses)
• Pregnancy (genetic and obstetric history, history of genital herpes, blood pressure, hemoglobin and hematocrit, ABO/Rh typing, RPR, HBsAg, GC culture, rubella serology, offer HIV screening)
• Family or personal history of skin cancer (complete skin exam)
• Obese patients with a family history of diabetes mellitus (fasting glucose)
Pablos-Mendez and Rabkin: Screening
• Women with history of gestational diabetes (fasting glucose)
• Fertile, sexually active women lacking evidence of rubella immunity (rubella serology)
• Patients with multiple sexual partners (RPR, GC and chlamydia culture, HIV testing
• Risk group for HIV (HIV testing)
• Household member of person with TB (PPD)
• Recent immigrant from tuberculosis-endemic area (PPD)
• Resident of a nursing home, correctional facility or homeless shelter (PPD)
• Family history of familial polyposis or colon cancer (colonoscopy, fecal occult blood test)
• Age over 65 (history of TIA symptoms, functional status at home, visual acuity, hearing exam)
Cervical Cancer Screening
New cases of invasive cervical carcinoma occur at an annual rate of approximately 20 per 100,000 in women over the age of 35. The lifetime incidence (in the absence of screening) is about 2,500 per 100,000 women, with about a 1 in 100 chance of dying from it. In screened populations, the epidemiology is quite different. While virtually undetected in unscreened populations, the additional annual incidence of carcinoma in situ (CIS) in screened populations is 130 per 100,000 with a lifetime incidence of 2,000 per 100,000. In a screened population where CIS is treated when it is detected, the lifetime incidence of invasive disease drops to about 700 per 100,000. Risk factors include race (blacks, Hispanics and Native Americans have a rate double that of whites and Asians), history of multiple sexual partners (two to three times higher risk), smoking (1.5 times higher risk) and oral contraceptives (1.2 to 2 times higher risk).
The standard screening procedure for cervical cancer is the Papanicolaou (Pap) test. It has many of the characteristics of a good screening test; it is safe, relatively inexpensive, and has a low false-positive rate (0.2 to 0.4 percent). Unfortunately, it has a fairly high false-negative rate (5 to 50 percent). However, given the relatively indolent nature of the disease (20 years from initial HPV infection to invasive cancer in women with intact immune systems), the impact of a single false negative is diminished considerably by periodic reexamination.
Cervical Cancer Screening
Pap smears at least every three years in women 20-65 who are or have been sexually active. In women over 65 with previously negative Pap smears, screening is no longer recommended.
Breast Cancer Screening
While lung cancer has surpassed breast cancer as the leading cause of cancer death in women, breast cancer remains the most commonly occurring cancer (after basal and squamous cell skin cancers) and remains one of the most common causes of death in women ages 35-55. Risk factors include family history of premenopausal breast cancer in a first degree relative (mother, sister or daughter), previous personal history of breast cancer or benign breast disease, menarche before age 12, first pregnancy after age 30, menopause after age 50, obesity, high socioeconomic
Breast cancer screening:
• Annual clinical breast examination
• Mammogram for women from 40 to 49 at the discretion of the physician and patient
• Annual mammogram for women age 50 to 74
• Mammogram for women over age 74 who have not previously been screened and annual or biannual mammogram at the discretion of the physician and patient
Colorectal Cancer Screening
Risk factors for colorectal cancer include family history, diet (high saturated fat, low fiber), alcohol use and sedentary life style. Like breast cancer, five-year survival is strongly dependent on the stage at which the disease is diagnosed, ranging from 90 percent for localized disease (Duke's A) to 6 percent for cases with distant metastases (Duke's D). Only one-third of cases are detected while still in the localized stage, suggesting that increased screening could result in improved overall mortality.
Screening procedures include digital rectal exam (DRE), fecal occult blood testing (FOBT) usually with Hemoccult cards, rigid sigmoidoscopy, flexible sigmoidoscopy (FOS), colonoscopy and biologic markers such as carcinoembryonic antigen (CEA). DRE is a useful method for conducting FOBT; however as a primary method for palpating colorectal lesions it is inadequate, since over 85 percent of lesions are beyond the reach of even the longest finger. FOS has generally replaced rigid sigmoidoscopy. Colonoscopy has not been extensively evaluated as a screening.
Prostate Cancer Screening
The Guide to Clinical Preventive Services summarizes studies of the effectiveness of digital rectal exam, transrectal ultrasonography and serum rumor markers (such as prostate specific antigen) for detection of and reducing mortality from prostate cancer. The Task Force concluded that there was no evidence to support screening for prostate cancer. The sensitivity and specificity of these tests is variable, and there is no evidence to support the hypothesis that early detection and aggressive treatment of prostate cancer leads to improved mortality.
Ovarian Cancer Screening
The Task Force concluded, based on the available data, that there is no evidence to support such screening. A NIH consensus panel concurred that ovarian cancer screening is not recommended.