Online Medical Reference

Principles of Screening

Raul J. Seballos

Published: August 2010

Preventive medicine encompasses primary, secondary, and tertiary preventive measures such as screening, counseling, immunization, and prophylaxis. The goal of primary prevention is to prevent new disease cases by reducing risk factors. Primary preventive measures are designed to prevent the onset of a targeted condition; they are activities that help avoid a specific health care problem. For example, smoking cessation can reduce the incidence of lung cancer. Other examples of primary prevention practices include immunization and prophylaxis.

The goal of secondary prevention is to detect disease or cancer, thereby leading to early treatment and improved prognosis. Secondary preventive measures identify and treat asymptomatic persons who already have certain risk factors or in whom the condition is not yet clinically apparent. Screening tests are examples of secondary preventive measures. With early case diagnosis, a disease's natural course, or how an illness unfolds over time without treatment, can often be altered to maximize a patient's well-being and minimize suffering. For example, a mammogram that detects an early-stage breast cancer can lead to a more definitive and curative procedure.

Tertiary prevention manages an existing disease, with the goal to restore a patient to highest function, minimize the negative consequences of the disease, and prevent disease-related complications. Examples include eliminating offending allergens from an asthmatic patient's environment and routinely screening for and managing early renal, eye, and foot problems in a diabetic patient.

An ideal screening test would be a reasonably priced, noninvasive procedure. This screening test must identify a disease that is clinically significant and that, if left untreated, will cause significant morbidity and mortality. In addition, the ideal screening test must be for a disease that has a preclinical phase, a presymptomatic stage for which the disease is detectable. Finally, the disease must have an acceptable treatment course.

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.

Table 1: Predictive Value of Test
Result Disease No Disease Total
Positive test True positive (TP) False positive (FP) Total patients with positive test (TP + FP)
Negative test False negative (FN)Total patients with disease True negative (TN)Total patients without disease Total patients with negative test (TN + FN)Total patients
Sensitivity = TP/(TP + FP) Positive predictive value = TP/(TP + FP)
Specificity = TN/(TN + FP) Negative predictive value = TN/(TN + FN)

The positive predictive value (PPV) is the ratio of patients with positive test results who are correctly identified to all patients who tested positive for the disease [PPV = TP/(TP + FP)] (where FP stands for false positive). A negative predictive value (NPV) is the ratio of patients with negative test results who are correctly identified to all patients who test negative for the disease [NPV = TN/(TN + FN)] (where TN stands for true negative and FN stands for false negative).

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

Table 2: Preventive Services Recommended by the USPSTF
Adults Special Populations


Condition Intervention Recommendations Adults Special Populations Men Women Pregnant Women Children
Abdominal aortic aneurysm Screening One-time screening by ultrasonography in men aged 65 to 75 who have ever smoked x
Alcohol misuse Screening and behavioral counseling x x x
Aspirin for primary prevention of cardiovascular events Prevention Adults at increased risk for coronary heart disease x x
Bacteriuria Screening for symptomatic infection x
Breast cancer Chemoprevention Discuss with women at high risk for breast cancer and at low risk for adverse effects of chemoprevention x
Breast cancer Screening Mammography every 1-2 yr for women ≥40 yr x
Breast and ovarian cancer susceptibility Genetic risk assessment and BRCA gene mutation testing Refer women whose family history is associated with an increased risk for deleterious mutations in BRCA1 or BRCA2 genes for genetic counseling and evaluation for BRCA gene testing x
Breast-feeding Behavioral interventions to promote Structured education and behavioral counseling programs x x
Cervical cancer Screening Women who have been sexually active and have a cervix x
Chlamydial infection Screening
  • Sexually active women ≤25 yr and other asymptomatic women at increased risk for infection
  • Asymptomatic pregnant women ≤25 yr and others at increased risk
Colorectal cancer Screening Men and women ≥50 yr x x
Dental caries in preschool children Prevention Prescribe oral fluoride supplementation at currently recommended doses to preschool children >6 mo whose primary water source is deficient in fluoride x
Depression Screening In clinical practices with systems to ensure accurate diagnoses, effective treatments, and follow-ups x x
Diabetes mellitus in adults, type 2 Screening Adults with hypertension or hyperlipidemia x x
Diet Behavioral counseling in primary care to promote a healthy diet Adults with hyperlipidemia and other known risk factors for cardiovascular and diet-related chronic diseases x x
Gonorrhea Screening All sexually active women, including those who are pregnant, at increased risk for infection (i.e., are young or have other individual or population risk factors) x x
Gonorrhea Prophylactic medication Prophylactic ocular topical medication for all newborns against gonococcal ophthalmia neonatorum x
Hepatitis B virus infection Screening Pregnant women at first prenatal visit x
High blood pressure Screening x x
HIV Screening All adolescents and adults at increased risk for HIV infection and all pregnant women x x x x
Lipid disorders Screening
  • Men ≥35 yr and women ≥45 yr
  • Younger adults with other risk factors for coronary disease
  • Screening for lipid disorders to include measurement of total cholesterol and HDL cholesterol
Obesity in adults Screening Intensive counseling and behavioral interventions to promote sustained weight loss for obese adults x x
Osteoporosis in postmenopausal women Screening Women ≥65 yr and women ≥60 yr at increased risk for osteoporotic fractures x
Rh(D) incompatibility Screening
  • Blood typing and antibody testing at first pregnancy-related visit
  • Repeated antibody testing for unsensitized Rh(D)-negative women at 24-28 wk gestation unless the biologic father is known to be Rh(D) negative
Syphilis infection Screening Persons at increased risk and all pregnant women x x x
Tobacco use and tobacco-caused disease Counseling to prevent
  • Tobacco cessation interventions for those who use tobacco
  • Augmented pregnancy-tailored counseling to pregnant women who smoke
Visual impairment in children <age 5 yr Screening To detect amblyopia, strabismus, and defects in visual acuity x

Note: The USPSTF recommends that clinicians discuss these preventive services with eligible patients and offer them as a priority. All these services have received an “A” (strongly recommended) or a “B” (recommended) grade from the Task Force.
HDL, high-density lipoprotein; HIV, human immunodeficiency virus; USPSTF, United States Preventive Services Task Force.
Adapted from Agency for Healthcare Research and Quality: The Guide to Clinical Preventive Services, 2006: Recommendations of the U.S. Preventive Services Task Force.

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Adult Immunizations

Immunization is an example of a primary preventive practice. The current Adult Immunization Schedule is available on the CDC's website.7 Detailed discussion of each of the recommended immunizations is discussed in the Infectious Disease section of this book.

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Summary

  • Preventive medicine encompasses primary, secondary, and tertiary preventive measures such as screening, counseling, immunization, and prophylaxis.
  • An ideal screening test would be a reasonably priced, noninvasive procedure that is highly sensitive and specific.
  • Numerous medical organizations have agreed that routine annual checkups for healthy adults should be abandoned in favor of a more selective approach to preventing and detecting health problems.

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References

  1. Centers for Disease Control and Prevention. State-specific prevalence of obesity among adults—United States, 2005. MMWR Morb Mortal Wkly Rep. 2006, 55: (36): 985-988.
  2. Ogden CL, Carroll MD, McDowell MA, Flegal KM. Obesity among adults in the United States—no change since 2003-2004. NCHS data brief no 1. Hyattsville, MD: National Center for Health Statistics, 2007. Available at http://www.cdc.gov/nchs/data/databriefs/db01.pdf (accessed March 20, 2009)
  3. Centers for Disease Control and Prevention. Data & Trends: Diabetes Surveillance System. 1999 Surveillance Report. Chapter 1: The Public Health Burden of Diabetes Mellitus in the United States.
  4. Oboler SK, Prochazka AV, Gonzales R, et al: Public expectations and attitudes for annual physical examinations and testing. Ann Intern Med. 2002, 136: 652-659.
  5. Agency for Healthcare Research and Quality. The Guide to Clinical Preventive Services, 2006: Recommendations of the U.S. Preventive Services Task Force.
  6. Reinhart D. Annual physical examination: Necessary or needless? Ann Intern Med. 2003, 138: (5): W-W48.
  7. Centers for Disease Control and Prevention. Recommendations and guidelines: Adult immunization schedule. Available at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5753a6.htm?s_cid=mm5753a6_e (accessed March 20, 2009)

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