Indications and Procedures (Magill’s Medical Guide, Sixth Edition)
In its natural course, a disease usually starts asymptomatically, gradually develops symptoms, and becomes severe when no treatment is given. If the disease can be identified early, especially before it shows any signs or symptoms, then treatment can be initiated early. Early or prompt treatment usually produces a better and more effective cure for a disease. Treatment at the early stage of a disease also has a lower cost than treatment for a disease at its late stages.
The most effective way to detect diseases before they develop symptoms is through screening. Screening is the application of a test or procedure to detect a potential disease or condition in individuals who have no known signs or symptoms of that disease or condition. For example, some people are unaware that they have a high cholesterol level, which can increase the risk for the development of cardiovascular disease. Through screening for cholesterol levels, these individuals can be identified, and proper dietary consultation, exercise prescription, and treatment can be followed to decrease the risk for cardiovascular disease. Screening tests use criteria to classify people according to the likelihood of disease. Screening is just an initial examination. Individuals with positive results from a screening test are required to have a secondary diagnostic examination, which is often more comprehensive and in depth, to confirm the positive finding from...
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Uses and Complications (Magill’s Medical Guide, Sixth Edition)
Some diseases are good candidates for screening. For example, data from the National Health and Nutrition Examination Survey showed that 24 percent of the adult population in the United States had high blood pressure, or hypertension. Almost one in every three African Americans was hypertensive. More than one-third of the individuals with hypertension were unaware of their condition, since hypertension is usually asymptomatic. Hypertension is defined as a blood pressure reading of 140/90 millimeters of mercury (mmHg) or higher. The higher the blood pressure, the higher is the chance of developing hypertension complications, such as coronary heart disease, stroke, renal dysfunction, and sudden death. The treatment of hypertension is very effective. Reduction in blood pressure through treatment can significantly decrease the risk for stroke, coronary heart disease, renal disease, and sudden death. Screening is the best way to detect hypertensive individuals without symptoms. The common method of screening for hypertension is sphygmomanometry that measures cuff pressures. Ambulatory blood pressure monitoring is an alternative method, which uses an automated sphygmomanometer that records blood pressure at frequent intervals over twenty-four hours. Although ambulatory blood pressure monitoring is more accurate, it is also more expensive and impractical, thus, it is not as commonly used as sphygmomanometry. As the benefit of the...
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Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
By the middle of the twentieth century, infectious diseases were gradually replaced by chronic diseases as the leading causes of death in developed countries such as the United States. This transition occurred as a result of improvement in living standards and nutritional status, the prevention of infectious diseases through immunization, and increased life expectancy. Hence, prevention of chronic disease became an important task in public health and medicine practices in developed countries. In 1951, the United States multi-sponsored Commission on Chronic Illness organized the Conference on Preventive Aspects of Chronic Diseases and advocated screening for diseases. Since then, more and more screening tests and procedures have become available for different diseases and conditions because of development in medical technologies.
At the beginning of the twenty-first century, however, chronic diseases were still the leading cause of death in developed countries as well as some developing countries. The most important chronic diseases that contribute to mortality are heart disease, cancer, and stroke. Many aspects of screening for these chronic diseases remain unclear. For some diseases, no tests or no effective tests are available, such as for lung cancer. For other conditions, powerful methods of detection exist but no effective treatments are available, such as for human immunodeficiency virus (HIV). Many screening...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
Holland, Walter W., and Susie Stewart. Screening in Disease Prevention: What Works? Seattle: Radcliffe, 2005. This book discusses different issues in screening for diseases in children, adults, and the elderly.
Snow, Vincenza, ed. Screening for Diseases: Prevention in Primary Care. Philadelphia: American College of Physicians, 2004. This book summarizes research findings of screening for major diseases, such as breast cancer, prostate cancer, colorectal cancer, hypertension, type 2 diabetes, and osteoporosis.
Thorner, Robert M., and Quentin R. Remein. Principles and Procedures in the Evaluation of Screening for Disease. Washington, D.C.: Government Printing Office, 1961. This classic monograph provides guidelines and methods for evaluating the effectiveness of screening programs.
U.S. Preventive Services Task Force. Guide to Clinical Preventive Services. Washington, D.C.: Agency for Healthcare Research and Quality, 2006. Offers guidelines for physicians, students in health fields, and the general public on the use of clinical preventive services. Contains detailed screening information for various health conditions.
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Screening (Encyclopedia of Public Health)
Screening is performed to identify the presence of a disease or a risk factor for a disease, typically among asymptomatic persons (those who do not already manifest symptoms of disease). In this way, a disease, or risk factors for a disease, can be detected early, allowing either early treatment or prevention, including preventing further spread of communicable or transmissible diseases. Screening tests are widely used by clinicians as part of the periodic health examination, as well as by public health officials. Examples of screening tests are as varied as blood tests to detect lead poisoning in young children, blood tests to detect the human immunodeficiency virus (HIV), measuring blood pressure to detect high blood pressure, mammography to detect breast cancer, sigmoidoscopy and colonoscopy to detect cancers of the rectum and colon, and questionnaires to identify persons with alcohol or other drug problems.
|Two-By-Two Table to Assess the Usefulness of a Screening Test|
|SOURCE: Courtesy of author.|
|Positive Test||True Positive (TP)||False Positive (FP)||Total Positive|
|Negative Test||False Negative (FN)||True Negative (TN)||Total Negative|
|Total with Disease||Total with No Disease|
Several factors determine the usefulness of a screening test for use with any individual person. The first is the accuracy of the test itself, specifically its sensitivity and specificity. Sensitivity is the probability that a person with the disease or risk factor will test positive. Specificity is the probability that a person without the disease or risk factor will test negative. Sensitivity and specificity are illustrated in Table 1. The sensitivity of a screening test is determined by the number of true positives divided by the total number with disease (or TP/[TP+FN]). The specificity is the number of true negatives divided by the total number with no disease (or TN/[FP+TN]).
Because there is often some overlap in the distributions of test results among people with and without disease (i.e., some people without disease will have test results in the disease range, and some people with disease will have test results in the no disease range), a test's sensitivity and specificity usually trade-off against one another. As the sensitivity increases the specificity usually decreases, and vice versa. A screening test that identifies almost all people with a disease (high sensitivity) may also produce more false positives among those people without the disease who may have borderline results (results near the cut-off value defined for the test). Conversely, a screening test that correctly identifies almost all people without the disease (high specificity) usually misses more people who truly have the disease (false negatives). Tables 2 and 3 represent the characteristics of two hypothetical screening tests when applied to a sample of 100,000 people with a true prevalence of disease of 10 percent (e.g., a relatively common disease). Table 2 is for a test with a sensitivity of 95
|Screening Test with High Sensitivity (95%) and Moderate Specificity (65%) in a Sample with a 10% True Prevalence of Disease|
|SOURCE: Courtesy of author.|
percent and a specificity of 65 percent. Table 3 is for a test with 65 percent sensitivity and 95 percent specificity. The test with high sensitivity (Table 2) identifies more people who truly have the disease and misses fewer people who truly have the disease (false negatives). However, this test incorrectly classifies more than three people without the disease (false positives) for every one person it correctly identifies with the disease. In contrast, the test with high specificity (Table 3) incorrectly classifies many fewer nondiseased people as having the disease (false positives) but misses more truly diseased people (false negatives).
In addition to the accuracy of the test itself, another important factor is how well the test is implemented. Errors may be introduced that depend on who is performing the test or on variations in the way the test is performed. For example, not all radiologists are equally proficient at reading mammograms and not all laboratories will get the same result when measuring cholesterol levels from the same blood sample. Therefore, the test characteristics that are initially reported for a screening test often represent a best case scenariohe best that a test can be expected to perform. As a result, it is also important to evaluate test accuracy in the real world settings where the tests are being used.
The usefulness of a screening test also depends upon the probability that the individual being tested has the disease or risk factor of interest. This is termed the "prior probability" of disease. This issue is illustrated by comparing Table 2 and Table 4. Table 4 represents the same hypothetical test shown in Table 2, but applied to a
|Screening Test with Moderate Sensitivity (65%) and High Specificity (95%) in a Sample with a 10% True Prevalence of Disease|
|SOURCE: Courtesy of author.|
sample in which the true prevalence of disease is less common, only 1 percent instead of 10 percent. As shown, screening tests are more useful when they are used on people who are more likely to have the disease than people who are less likely to have the disease. When a screening test is used in a sample with a lower prior probability of disease, even more false positives are identified. In this example (Table 4), the test has incorrectly identified more than 36 people who do not really have the disease (false positives) for every one person correctly identified with the disease (true positives).
Prior probability is taken into account in calculating the predictive value of a test. The predictive value of a positive test is the probability that someone who tests positive truly has the disease. For the examples shown in Tables 2, 3, and 4, the predictive values of a positive test are 23 percent, 59 percent, and 3 percent, respectively. As can be seen, the predictive value of a positive test is increased when tests with higher specificity are used in samples of people with a higher prevalence of the disease.
Finally, the usefulness of a screening test depends on the existence of an effective and feasible treatment. This may include treatment for the disease or risk factor detected, and/or an intervention to prevent further spread of the problem to others, such as removing lead paint from homes or genetic counseling. If there are no feasible and effective responses to the results of a screening test (e.g., the result wouldn't change anything) then there is no reason to perform the test.
These issues are of particular concern for screening asymptomatic or healthy people. All
|Screening test with high sensitivity (95%) and moderate specificity (65%) in a sample with a 1% true prevalence of disease|
|SOURCE: Courtesy of author.|
testing involves risks. These risks might be acceptable to the small number of persons who turn out to have the disease. However, the risks of side-effects from the screening tests themselves, or from an incorrect or ambiguous diagnosis and the subsequent testing that an incorrect initial test result requires, may not be acceptable to the much larger number of people who do not have the disease or risk factor of interest. In addition, there are economic costs to screening large numbers of asymptomatic people to identify a small number of people with disease. Therefore, clinicians, patients, and public health professionals must weigh the risks and benefits when deciding to use a screening test for any individual or population.
THOMAS N. ROBINSON
(SEE ALSO: Assessment of Health Status; Blood Lead; Blood Lipids; Breast Cancer; Cancer; Cholesterol Test; Colorectal Cancer; Diabetes; HIV/AIDS; Mammography; PAP Smear; Periodic Health Examination; Prevention; Preventive Medicine; Serological Markers; VDRL Test)
Kraemer, H. C. (1992). Evaluating Medical Tests: Objective and Quantitative Guidelines. Newbury Park, CA: Sage Publications.
U.S. Preventive Services Task Force (1996). Guide to Clinical Preventive Services, 2nd edition. Washington, DC: U.S. Department of Health and Human Services.