There are scores of diagnostic tests—blood tests, x-rays, computed tomography (CT) scans, ultrasounds, magnetic resonance imaging (MRI)—to help the health care practitioner identify the cause of disease. It is important for practitioners to choose tests that not only improve their understanding of the disease but also affect treatment decisions. The decision to order a specific diagnostic test takes into account the test's reliability, validity, sensitivity, and specificity in addition to its risks to the patient and costs in terms of time and dollars.
The Reliability and Validity of Diagnostic Tests
Reliability of diagnostic testing refers to the test's ability to be repeated and to produce equivalent results in comparable circumstances. A reliable test is consistent and measures the same way each time it is used with the same patients in the same circumstances. For example, a well-calibrated balance scale is a reliable instrument for measuring body weight.
Validity is the accuracy of the diagnostic test. It is the degree to which the diagnostic test measures the disease, blood level, or other quality or characteristic it is intended to detect. For example, a good diagnostic test reliably distinguishes those who have the disease from those who do not. There are two components of validity—sensitivity and specificity.
THE SENSITIVITY AND SPECIFICITY OF DIAGNOSTIC TESTS.
Sensitivity refers to a test's ability to identify people who have the disease. Specificity, on the other hand, refers to a test's ability to identify people who do not have the disease. Ideally, diagnostic tests would be highly sensitive and highly specific, thereby accurately classifying all people tested as either positive or negative. In practice, however, sensitivity and specificity are frequently inversely related—most tests with high levels of sensitivity have low specificity, and the reverse is also true.
The likelihood that a test result will be incorrect can be gauged based on the sensitivity and specificity of the test. For example, if a test's sensitivity is 95 percent, then when one hundred patients with the disease are tested, ninety-five will test positive and five will test "false negative"—they have the disease but the test has failed to detect it.
However, if a test is 90 percent specific, when one hundred healthy, disease-free people are tested, ninety will receive negative test results and ten will be given "false-positive" results—they do not have the disease but the test has inaccurately classified them as positive.
The advantages of highly sensitive tests are that they produce few false-negative results, and people who test negative are almost certain to be truly negative. Highly sensitive tests may be useful as preliminary screening measures for diseases where early detection is vitally important, such as the enzyme-linked immunosorbent assay (ELISA) screening test for human immunodeficiency virus (HIV), the virus that produces acquired immune deficiency syndrome (AIDS).
In contrast, highly specific tests produce very few false-positive results and those who test positive are nearly certain to be positive. Highly specific tests are useful when confirming a diagnosis and in cases where the risks of treatment are high, such as the Western blot test to confirm the presence of HIV after it has been detected by the highly sensitive, but less specific, ELISA test.
Laboratory Tests
The editors of the fifteenth edition of Harrison's Principles of Internal Medicine observe that the increasing number and availability of laboratory tests has encouraged physicians and other health care practitioners to grow increasing reliant on them as diagnostic tools. Laboratory tests are easy, convenient screening measures because multiple laboratory tests may be performed on a single sample of blood and abnormal laboratory test results can provide valuable clues for diagnosis.
For screening purposes (to detect disease at its earliest stage, before it produces symptoms), the physician or other health care practitioners may order a variety of blood tests, based in part on the patient's age, gender, and medical history. These tests may measure the following:
- Fasting blood sugar—this test is a screening and diagnostic test for diabetes; values consistently greater than 126 mg/dl indicate diabetes
- Calcium—blood levels of calcium can be elevated as a result of hyperactive parathyroid glands
- Lipids—elevated cholesterol, triglycerides, and low-density lipoproteins are associated with increased risk of heart disease
- Thyroid stimulating hormone (TSH)—high levels of TSH indicate hypothyroidism (underactivity of the thyroid gland), and abnormally low levels indicate hyperthyroidism (overly active thyroid gland)
- VDRL (Venereal Disease Research Laboratory) or RPR (rapid plasma reagin)—these tests screen for syphilis, a sexually transmitted disease
- HIV—screening for the presence of the virus that causes AIDS
- PSA (prostate specific antigen)—this blood test is used to screen for prostate cancer and to monitor treatment of the disease
- Stool occult blood (also called fecal occult blood test)—tests for the presence of blood in the stool, which could be an indicator of colon cancer
Diagnostic Imaging Techniques
Imaging studies are another form of diagnostic testing. In the past, all diagnostic imaging studies were obtained using ionizing radiation (x-rays) and recorded on transparent film. Modern imaging studies such as ultra-sound and magnetic resonance imaging (MRI) use nonionizing radiation and can be recorded digitally, viewed on computer monitors, sent via electronic mail, and stored on compact discs, digital tape, or transparent film.
Imaging studies are performed by trained radiology technologists and technicians, and they are read and interpreted by radiologists (physician specialists with advanced training in diagnostic imaging). Most imaging studies are painless and pose little risk to patients apart from minimal exposure to radiation.
X-RAYS AND ULTRASOUND.
The images produced by x-rays are the result of varying radiation absorption rates of different body tissues—the calcium in bone has the highest x-ray absorption, soft tissue such as fat absorbs less, and air absorbs the least. Chest x-rays, which offer images of the lungs, ribs, heart, and diaphragm, are among the most frequently ordered imaging studies.
To view tissues normally invisible on x-ray, contrast agents, such as barium and iodine, may be introduced into the body. For example, contrast agents often are used for imaging studies of the gastrointestinal tract (GI) to diagnose digestive disorders. An "upper GI series" is a test that looks at the esophagus, stomach, and small intestine. After the patient swallows a liquid containing barium, these organs are visible on x-rays and may be examined for the presence of inflammation, ulcers, and cancer.
Another common use of diagnostic x-rays is the measurement of bone density. Bone mass measurement (also called bone mineral density) is performed to evaluate the risk of bone fractures. Bone density usually is measured in the spine, hip, and/or wrist because these are the most common sites of fractures resulting from osteoporosis, a disease in which bones become weak, thin, fragile, and more likely to break. Women older than age 65 are at the greatest risk for osteoporosis.
Mammography also relies on x-ray technology to detect and pinpoint changes or abnormalities in the breast tissue that are too small to be felt by hand. Another imaging technique for breast examination is ultrasound, which can accurately distinguish solid tumors (lumps or masses) from fluid-filled cysts.
Ultrasound images are produced using the heat reflected from body tissues in response to high-frequency sound waves. Whereas x-ray is ideal for examining bone, ultrasound is used to examine soft tissue, such as the ovaries, uterus, breast, and prostate. It is not suitable for looking at bones because calcium-containing tissues such as bone absorb, rather than reflect, sound waves.
COMPUTED TOMOGRAPHY, MAGNETIC RESONANCE IMAGING, AND POSITRON EMISSION TOMOGRAPHY.
For conventional flat x-rays, the patient, x-ray source, and camera remain fixed and immobile. Computed tomography (CT) scans use a mobile x-ray source and generate a series of cross-section pictures, or slices, that are assembled by computer into images. Because CT distinguishes differences in soft tissue more effectively and with higher resolution than conventional x-rays, it often is used to examine internal organs in the abdomen, such as the liver, pancreas, spleen, kidneys, and adrenal gland, and the aorta and vena cava (large blood vessels that pass through the abdomen).
Magnetic resonance imaging (MRI) generates images based on interaction between a large magnet, radio waves, and hydrogen atoms in the body. Stimulated by ordinary radio waves within the powerful magnetic field, these atoms give off weak signals that a computer builds into images. MRI frequently is used to create images of the brain, spinal cord, heart, abdomen, bone marrow, and knee.
CT and MRI scans generate images of the body's structure (anatomy), whereas positron emission tomography (PET) scans offer insight into body function or processes (physiology). To create PET images, positron-emitting atoms are injected into the body, where they travel and strike other electrons, producing gamma rays. The gamma rays then are interpreted into images by a computer.
Unlike CT and MRI, PET rarely is used for screening or diagnostic purposes. Instead, it is used to track the progress and treatment of patients with diagnosed diseases such as cancer.
Diagnostic Procedures
Other diagnostic tests commonly performed to screen for the presence of disease include the following:
- Throat culture—This test is used to determine whether streptococcus pyrogenes (commonly called strep) bacteria are the cause of a sore throat. To obtain a sample of the mucus in the throat, the health care practitioner swabs the back of the throat and places the swab in a tube. The swab is transferred into a culture in the laboratory where it is examined for bacterial growth. The results of this test are available in two to three days. A "rapid" strep test that produces results in minutes is also available.
- Urinalysis and urine culture—Chemical and microscopic examination of urine allow identification of infection, diabetes, and the presence of blood in the urine.
- Colonoscopy—Using a long tube fitted with a lens, the health care practitioner is able to look at the entire colon; identify and remove polyps; detect cancer; and diagnose other causes of blood in the stool, abdominal pain, and digestive disorders. To prepare for colonoscopy, patients must empty the intestines completely before the examination.
- Flexible sigmoidoscopy—This test is similar to the colonoscopy in its use of a tube fitted with a camera to examine the colon. However, because the instrument is shorter than a colonoscope, it does not enable views of the entire colon. Through the flexible sigmoidoscope, the practitioner can examine only the sigmoid (lower portion) of the colon to detect polyps and cancers.
- Electrocardiogram—This test assesses the electrical function of the heart, detects abnormal heart rhythms, and aids in the diagnosis of myocardial infarction (heart attack) and other heart diseases.
Prenatal Diagnostic Testing
Ultrasound is used routinely to monitor the progress of pregnancy; evaluate the size, health, and position of the fetus; and detect some birth defects. Fetal ultrasound assists in the prediction of multiple births (more than one baby) and sometimes provides information about the gender of the unborn child.
Chorionic villus sampling (CVS) enables obstetricians and perinatologists (physicians specializing in evaluation and care of high-risk expectant mothers and babies) to assess the progress of pregnancy during the first trimester (the first 3 months). A physician passes a small, flexible tube called a catheter through the cervix to extract chorionic villi tissue—cells that will become the placenta and are genetically identical to the baby's cells. The cells are examined in the laboratory for indications of genetic disorders such as cystic fibrosis, Down syndrome, Tay-Sachs disease, and thalassemia. The results of the testing are available within seven to fourteen days. CVS provides the same diagnostic information as amniocentesis; however, the risks (miscarriage, infection, vaginal bleeding, birth defects) associated with CVS are slightly higher.
Amniocentesis involves analyzing a sample of fluid that surrounds the fetus in the uterus. The fluid is obtained when a physician inserts a hollow needle through the abdominal wall and the uterine wall. Like CVS, amniocentesis samples and analyzes cells derived from the baby to enable parents to learn of chromosomal abnormalities and the gender of the unborn child. Results usually are available about two weeks after the test is performed.
Blood tests are also available to help diagnose fetal abnormalities. The enhanced alpha-fetoprotein test (also called a triple screen) measures levels of protein and hormones produced by the fetus and can identify some birth defects, such as Down syndrome and neural tube defects. Two of the most common neural tube defects are anencephaly (absence of the majority of the brain) and spina bifida (incomplete development of the back and spine). Test results are available within two to three days. Women with abnormal results often are advised to undergo additional diagnostic testing, such as CVS or amniocentesis.
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