What are medical screenings?

Indications and Procedures

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 their screening test.

Screening tests can be classified into three categories: mass, multiple or multiphasic, and prescriptive. Mass screening means the screening of all individuals in a defined area or population. Multiple or multiphasic screening involves the use of a variety of tests or procedures during the same screening visit. Prescriptive screening is the early detection in presumptively healthy individuals of disease that can be controlled better if treated early in its natural history. From a public health perspective, the following factors are of importance to consider before implementing a screening program. The disease or condition should be a major medical problem. Effective treatment should be available for the disease discovered from the screening test. Follow-up diagnosis and treatment should be available to individuals with positive results from the screening test. The disease should have identifiable early and latent stages. An effective test or examination for the disease should be available. The test or procedure used in screening should be acceptable to the general population. The natural history of the disease should be adequately understood. Policies, procedures, and threshold levels on tests should be determined in advance to clarify who should be referred for further
diagnosis and possible treatment. The process should be simple enough to encourage large groups of people to participate. Finally, screening should be done as a regular and ongoing process, not as an occasional activity. Based on these considerations, screening is not necessarily available or suitable for every disease.

Accuracy and effectiveness are important for a screening test; they are evaluated by the characteristics of validity, predictive value, reliability, and yield. The validity of a test means how well the test actually measures what it is supposed to measure. Validity has two components: sensitivity and specificity. Sensitivity is defined as the ability of a test to identify correctly those who have the disease, while specificity is defined as the ability of a test to identify correctly those who do not have the disease. Both sensitivity and specificity are determined by comparing the results obtained from the screening test with the results derived from some definitive diagnostic procedure. Sensitivity is calculated as the proportion of subjects with the disease who have a positive screening test (true positive). Specificity is calculated as the proportion of subjects without the disease who have a negative screening test (true negative). If a screening test has a low sensitivity, then it could misclassify healthy individuals as disease cases (false positive). If a screening test has a low specificity, then it could fail to identify individuals with the disease (false negative). Thus, high levels of sensitivity and specificity are the foundation
for the accuracy of a screening test, although it is impossible to have 100 percent of both sensitivity and specificity in practice.

The ability of a screening test to predict the presence or absence of the disease indicates the quality of the test. The predictive value of a screening test is influenced by the sensitivity and specificity of the test as well as the prevalence of the disease, which means how common the disease is in populations. The higher the prevalence of a disease in a population, the more likely a positive test will represent a true positive; the lower the prevalence of a disease in a population, the more likely a positive test will represent a false positive. The predictive value of a screening test can be measured as either positive or negative. The predictive value of a positive test is the probability that individuals with a positive test actually have the disease. On the other hand, the predictive value of a negative test is the probability that individuals who have a negative result from a screening test do not have the disease. Thus, along with sensitivity and specificity, a high level of predictive value is also desirable for an effective screening test. In comparison, sensitivity and specificity are calculated from the screening test results of the diseased or nondiseased individuals, while predictive value depends on the proportion of diseased individuals in the population.

Reliability, also called precision, is another important characteristic of a screening test. A reliable screening test is one that gives consistent results when the test is performed more than once on the same individual under the same conditions. Two common factors affect the consistency of results: the variation of the method used in a screening test and the variation between different observers. These variations usually can be reduced by standardization of procedures or protocols used in a screening test, by training the observers of a screening test, by using more than one observer at the same time with each making independent observations, and by quality control measures, such as periodic checks.

The yield of a screening program is defined as the number or proportion of cases of a previously unrecognized disease that are diagnosed and brought to treatment as a result of the screening program. Several factors can affect the yield of a screening program. If a screening test has a low sensitivity, and therefore identifies only a portion of the diseased individuals, then the yield of the screening test could be poor. If the prevalence of the disease being screened for is low, then the yield of the test could be low even though the sensitivity of the test is high. Using multiphasic screening approaches often increases the cost-effectiveness of a screening program and results in a high level of yield. Frequency of screening is another factor influencing the yield of a screening program. The optimal interval between screenings should be determined by a particular disease’s natural course—how long it takes to develop from its first detectability of symptoms—and by different individuals with different risk factors. For example, certain cancers may need to be screened more frequently in older individuals than in younger individuals. The yield of a screening program can also be affected by the proportion of the population participating in the program. Low participation rates could turn out low yields. To increase
participation rates in a disease screening program, the population needs to be educated about the disease, including information about the fact that the disease is understood as a serous threat to health, that all individuals are vulnerable to the disease, that the method of the screening test is safe, and so on. The cost of a screening test can also have a significant effect on the participation rate.

Screening is usually concerned with chronic illness. Physicians may prescribe specific screening tests based on the age, gender, occupation, overall health status, lifestyle, and medical and family history of the individuals being tested. Mass screening programs may be carried out in communities to detect common diseases such as hypertension, high blood cholesterol, type 2 diabetes, breast cancer, colorectal cancer, prostate cancer, and osteoporosis.

Uses and Complications

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 screening is evident and the risk of sphygmomanometry is nearly none, it is recommended that every adult should be screened for hypertension.

High blood cholesterol is another common condition that is asymptomatic but increases the risk of coronary heart disease and stroke. More than 37 percent of people in the United States have blood cholesterol levels higher than the normal range. Cholesterol comes from food or is produced in the body. When there is too much cholesterol in the blood, it builds up in the walls of the arteries and hardens them, so that the arteries become narrowed and blood flow to the heart or the brain is slowed down or blocked, resulting in coronary heart disease or stroke. Since high cholesterol is asymptomatic, many people are unaware that their level is too high. Cholesterol screening is performed using a blood test after a fourteen-hour fast. This test includes measurements of total cholesterol: both low-density lipoproteins (LDLs), or “bad” cholesterol, the main source of cholesterol buildup and blockage in the arteries, and high-density lipoproteins (HDLs), or “good”
cholesterol, which helps keep cholesterol from building up in the arteries. Triglyceride, another form of fat in the blood, may be checked at the same time. A healthy individual should have a total cholesterol level less than 200 milligrams per deciliter, an LDL level less than 100 milligrams per deciliter, and an HDL level greater than 45 milligrams per deciliter. The normal triglyceride level is less than 200 milligrams per deciliter. Individuals aged twenty years or older are recommended to have their cholesterol level checked every five years. Cholesterol screening can raise awareness of high blood cholesterol, so that individuals can modify their lifestyle by consuming less saturated fat and engaging in more physical activity and drug treatment can be initiated, if necessary, to prevent heart disease and stroke. The American Heart Association recommends mass screening for high cholesterol at worksites to identify high-risk populations of middle-aged men and women and in communities targeting low-income, low-education, and minority groups who are often underrepresented in other voluntary screenings.

The number of people with type 2 diabetes is increasing in the United States as a result of the high prevalence of obesity. Complications of diabetes are severe, such as increased risks for cardiovascular disease, kidney failure, blindness, and amputation of the legs. Many people with diabetes are undiagnosed. The natural history of diabetes includes an asymptomatic preclinical phase. The length of this preclinical phase varies and may last ten years or longer. Screening tests can detect diabetes in the preclinical phase. Different tests have been used for diabetes screening: the two-hour postload plasma glucose test, the fasting plasma glucose test, and the test of hemoglobin A1c. The third test has been proposed as a standard reference for diagnosing diabetes. Research studies showed that after diabetes was detected during its preclinical phase, interventions, such as aggressive control of hypertension and cholesterol levels and use of aspirin during the preclinical phase, could reduce the risk of cardiovascular disease. The benefit of tight control of blood sugar levels during the preclinical phase,
however, was unclear or minimal. Overall, screening for diabetes has been controversial. The most important gap in the understanding of screening is the lack of information on the added benefits of starting various interventions and treatments earlier, during the preclinical period, compared with at clinical detection.

More than 215,000 women are diagnosed with breast cancer
every year in the United States. Approximately one in every eight women will develop breast cancer during her lifetime. Breast cancer is the second leading cause of cancer death in women. Early detection and effective treatment can reduce the number of women who die from breast cancer. Three approaches are considered for screening for breast cancer: breast self-examination (BSE), x-ray mammography, and clinical breast examination. The sensitivity of BSE and clinical breast examination is unclear, but the sensitivity of mammography is relatively high. The American Cancer Society recommends screening with mammography every one to two years, with annual clinical breast examinations beginning at the age of forty and annual mammography with clinical breast examinations beginning at the age of fifty.

Prostate cancer is the most common cancer and the second leading cause of cancer death in men in the United States. After fifty, the risk of prostate cancer increases with age. African American men have a higher risk for prostate cancer. The principal screening tests for prostate cancer are the digital rectal examination, a blood test of serum
tumor markers called prostate-specific antigen (PSA), and transrectal ultrasound. The reference standard for these tests is pathologic confirmation of malignant disease in tissue obtained by biopsy or surgical resection. Because biopsies are not generally performed on patients with negative screening test results, the sensitivity and specificity of screening tests for prostate cancer cannot be determined with certainty. The American Cancer Society
recommends an annual digital rectal examination for prostate cancer beginning at age forty. It recommends an annual measurement of serum tumor markers for average men aged fifty and older; however, this measurement should begin at age forty for African American men and those with a family history of prostate cancer. The natural history of prostate cancer is still poorly understood, and there is no direct evidence that prostate cancer screening decreases mortality. The cost associated with the screening tests for prostate cancer is another concern. Thus, screening for prostate cancer remains controversial.

Colorectal cancer is the second most common form of cancer, as well as one of the leading causes of death from cancer in the United States. Since it is asymptomatic at its early stage, about 60 percent of patients with colorectal cancer have regional or distant metastases at the time of diagnosis. The principal screening tests for detecting colorectal cancer are fecal occult blood testing,
sigmoidoscopy, and digital rectal examination. Less frequently used screening tests include barium enema and colonoscopy, which have been advocated primarily for high-risk groups, such as persons with a family history of hereditary syndromes associated with a high risk of colon cancer. Screening for colorectal cancer is recommended for men and women aged fifty years and older with annual fecal occult blood testing, sigmoidoscopy,
or both. There is good evidence that periodic fecal occult blood testing reduces mortality for colorectal cancer, and there is fair evidence that sigmoidoscopy alone or in combination with the fecal occult blood testing reduces mortality. There is no direct evidence, however, that colonoscopy or barium enemas reduce mortality. The digital rectal examination is of limited value as a screening test for colorectal cancer. The examining finger, which is only seven to eight centimeters long, has limited access even to the rectal mucosa, which is usually eleven centimeters in length. Thus, a negative digital rectal examination performed by a physician provides little reassurance that the patient is free of colorectal cancer.

It is estimated that in the United States 1.3 million people experience a fracture each year as a result of osteoporosis, or fragile bones. Osteoporosis can progress painlessly until a bone breaks. Over half of all postmenopausal women will develop a spontaneous fracture as a result of osteoporosis. Hip fractures and vertebral deformities, two common consequences of osteoporosis, can result in significant pain, disability, decreased functional independence, and death. Low bone density is strongly associated with an increased risk of fracture. The World Health Organization (WHO) defines osteoporosis as a bone mineral density of 2.5 or more standard deviations below the mean of healthy young adult women. A number of tests are available to measure bone density, including conventional skeletal radiographs, quantitated computed tomography, single photon absorptiometry, dual photon absorptiometry, and dual energy x-ray absorptiometry. The correlations among different bone density devices are low. Dual energy x-ray absorptiometry is considered the gold standard because it is the most extensively validated test against fracture outcomes. The likelihood of being diagnosed with osteoporosis depends on the number of tested sites, which include the forearm, hip, spine, or heel. Because women over age sixty-five have a significantly increased risk of osteoporosis, routine screening in this population is recommended. For women aged sixty to sixty-four, screening may be needed if they have risk factors, such as low body weight, family history, smoking, and removal of the ovaries before age forty-five. The screening intervals should be two years for women aged sixty-five years and older and five years for women younger than sixty-five. Once osteoporosis is diagnosed, changing lifestyle, such as an increase in dietary calcium and vitamin D intake, weight-bearing exercise, and smoking cessation, along with drug treatment, can help prevent fractures.

Other common screening procedures include those meant to detect cervical cancer, testicular cancer, skin cancer, carotid artery stenosis, peripheral arterial disease, abdominal aortic aneurysm, thyroid disease, iron deficiency anemia, sexually transmitted diseases, visual impairment, hearing impairment, Phenylketonuria (PKU), Down syndrome, dementia, and depression.

Perspective and Prospects

By the middle of the twentieth century, infectious diseases were gradually replaced by chronic diseases as the leading causes of death in developed countries. 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 multisponsored 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. Many screening procedures still lack evidence to support their effectiveness or justification for the balance between benefit, harm, and cost. Therefore, further investigations, epidemiological studies, development of low-cost and effective screening methods, and increases in participation in screening through health education are the future challenges.

Bibliography

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U.S. Preventive Services Task Force. Guide to Clinical Preventive Services. Washington, D.C.: Agency for Healthcare Research and Quality, 2006.