CHAPTER 3
DIAGNOSING DISEASE: THE PROCESS OF IDENTIFYING THE CAUSES OF ILLNESS

Diagnosis means finding the cause of a disorder, not just giving it a name.

—Sydney Walker III

The practice of medicine often is considered to be both science and art because identifying the underlying causes of disease and establishing a diagnosis require that health care practitioners use a combination of scientific method, intuition, and interpersonal (communication and human relations) skills. Diagnosis relies on the powers of observation; listening and communication skills; analytical ability; knowledge of human anatomy (structure and parts of the human body) and physiology (the functions and life processes of body systems); and an understanding of the natural course of illness.

The editors of the fifteenth edition of Harrison's Principles of Internal Medicine (edited by Eugene Braunwald et al., McGraw-Hill, New York, 2001) explain that diagnosis requires a logical approach to problem-solving involving analysis and synthesis. In other words, health care practitioners must systematically break down the information they obtain from a patient's medical history, physical examination, and laboratory test results and then reassemble it into a pattern that fits a well-defined syndrome (a group of symptoms that collectively describe a disease).

HISTORY OF A PATIENT'S HEALTH AND ILLNESS

Obtaining a complete and accurate medical history is the first step in the diagnostic process. In fact, many health care practitioners (physicians, nurses, and allied health professionals) believe that the patient's medical history is the key to diagnosis and that the physical examination and results of any diagnostic testing (laboratory analyses of blood or urine, x-rays, or other imaging studies) simply serve to confirm the diagnosis made on the basis of the medical history.

A medical history is developed using data collected during the health care practitioner's interview with the patient. The medical history also may include data from a health history form or health questionnaire completed by the patient before the visit with the practitioner. The objectives of taking a medical history are as follows:

• Obtain, develop, and document (create a written record) a clear, accurate, chronological account of the individual's medical history (including a family history, employment history, social history, and other relevant information) and current medical problems.
• List, describe, and assign priority to each symptom, complaint, and problem presented.
• Observe the patient's emotional state as reflected in voice, posture, and demeanor.
• Establish and enhance communication, trust, understanding, and comfort in the physician–patient (or nurse–patient) relationship

In addition to eliciting a history of all of the patient's previous medical problems and illnesses, the health care practitioner asks questions to learn about the history of the present illness or complaint—how and when it began, the nature of symptoms, aggravating and relieving factors, its effect on function, and any self-care measures the patient has taken.

The medical history also includes a review of physiological systems—such as the cardiovascular (related to heart and circulation), gastrointestinal (GI; digestive disorders), psychiatric (mental and emotional health), and neurologic systems (brain and nerve disorders)—through which the patient may experience symptoms of disease. The review of systems frequently helps the practitioner obtain information to help assess the severity of the present problem and confirm the diagnosis.

Because it relies on the patient's assessment of the severity, duration, and other characteristics of symptoms, as well as the patient's memories and interpretation of past illnesses, the medical history provides the practitioner with subjective information. Together with the objective findings of the physical examination and other diagnostic tests, it helps practitioners to identify disease correctly.

PHYSICAL EXAMINATION

The U.S. National Library of Medicine defines physical examination as "the process of examining the patient's body to determine the presence or absence of physical problems." It includes inspection (looking), palpation (feeling), auscultation (listening), and percussion (tapping to produce sounds).

Vital Signs

In a clinic or office-based medical practice, the physical examination may begin with a nurse or medical assistant measuring the patient's vital signs—temperature, respiration, pulse, and blood pressure. Temperature is measured using a thermometer. Normal oral temperature (measured by mouth) is 98.6 degrees Fahrenheit or 37 degrees Celsius. Temperature also may be measured rectally, under the arm (axillary), or aurally with an electronic thermometer placed in the ear. Temperatures measured aurally or rectally are normally higher (approximately 99.6 degrees F/37.7 degrees C) than axillary or oral temperatures.

Respiration is measured by observing the patient's rate of breathing. In addition to determining the rate of respiration (normal for an adult is twelve to twenty breaths per minute), the practitioner also notes any difficulties in breathing.

Pulse rate and rhythm are assessed by compressing the resting patient's radial artery at the wrist. The normal resting pulse rate is between sixty and one hundred beats per minute, and the rhythm should be regular, with even spaces between beats. Pulse rates higher than one hundred beats per minute are called tachycardia, and rates lower than sixty beats per minute are called bradycardia. Some variations in pulse rates are considered normal and do not signify disease. Athletes who engage in high levels of physical conditioning often have pulse rates of less than sixty beats per minute at rest. Similarly, pulse rates increase naturally in response to exercise or emotional stress.

Blood pressure is measured using an inflatable blood pressure cuff, also known as a sphygmomanometer. The cuff is wrapped around the patient's upper arm, slightly above the elbow level, and as it deflates, the practitioner uses a stethoscope to listen to beats of the brachial artery. Blood pressure is measured in millimeters of mercury (mm Hg). Two readings are recorded—systolic pressure is the top number of a blood pressure reading and represents the pressure at which beats are first heard in the artery. The bottom number is the pressure at which the beats can no longer be heard; it is called diastolic pressure.

As with pulse rates, blood pressure varies in response to exercise and emotional stress. Normally, the systolic blood pressure of an adult is less than 140 mm Hg and diastolic blood pressure is less than 90 mm Hg. Repeated blood pressure readings higher than 140/90 mm Hg lead to a diagnosis of hypertension (high blood pressure).

Head and Neck

Physical examination of the head and neck involves inspection of the head (including skin and hair), ears, nose, throat, and neck. An instrument called an otoscope is used to examine the ear canal and tympanic membrane. Ears and nose are examined for swelling, redness, lesions, drainage, discharge, or deformity. Inspecting the throat, the practitioner looks for abnormalities and, by depressing the tongue, can inspect the mouth, oropharynx, and tonsils.

The practitioner notes any scars, asymmetry, or masses (lumps or thickenings) in the neck and systematically palpates (presses) to examine the chains of lymph nodes (also known as "lymph glands," clusters of cells that filter fluid known as lymph) that run in front and behind the ear, near the jaw, and at the base of the neck. The practitioner also inspects and palpates the thyroid gland (the largest gland in the endocrine system, located where the larynx and trachea meet).

Eye Examination

An eye examination consists of a vision test and visual inspection of the eye and surrounding areas for abnormalities, deformities, and signs of infection. Two numbers describe visual acuity (vision). The first number is the distance (in feet) that the patient is standing from the test chart, and the second number is the distance from which the eye can read a line of letters from the test chart. Because 20/20 is considered normal vision, a person with 20/60 vision can read a line of letters from 20 feet away that a person with normal vision could read from a distance 60 feet away from the test chart.

Using an ophthalmoscope, the practitioner examines the inner structures of the eye by looking through the pupil.

Chest and Lungs

Examination of the chest and lungs focuses on identifying disorders of breathing by observing for structural symmetry, listening for abnormal breath sounds and unusual effort during breathing, and noting the rate and rhythm of breathing. Breathing consists of inspiration and expiration (inhaling and exhaling), and changes in the length of either action could be a sign of disease. For example, prolonged expiration may be the result of the airway obstruction of asthma.

Percussion is a tapping technique used to produce sounds on the chest wall that may be distinguished as normal, dull, or hyperresonant. Dull sounds may indicate the presence of pneumonia (infection of the lungs), whereas hyperresonant sounds may be signs of a collapsed lung (pneumothorax) or emphysema (a disease in which the alveoli—microscopic air sacs—of the lung are destroyed).

The practitioner listens to breath sounds with a stethoscope. Listening with the stethoscope is called auscultation. Decreased breath sounds may be signs of emphysema or pneumothorax, whereas high-pitched wheezes are associated with asthma. Another device used to monitor the breathing of patients with asthma is a peak flow meter. After taking a deep breath, the patient exhales into the peak flow meter and it measures the velocity of exhaled breath.

Back and Extremities

The examination of the back and extremities (arms and legs) focuses on the anatomy of the musculoskeletal system. The practitioner looks for symmetry, swelling, color (redness may be a sign of infection; blueness may indicate inadequate blood supply), deformity, and loss of function. Major muscle groups and all joints are examined. The upper-body extremities include the fingers, thumbs, wrists, forearms, elbows, and shoulders. The lower extremities consist of hips, knees, ankles, feet, and toes. All joints and the spine are observed as the patient moves to determine stability and range of motion, and each is palpated to watch for pain, tenderness, or weakness.

Pulses on the arms, legs, and feet (radial, posterior tibial, and dorsalis pedis respectively) are checked to be certain blood flow to the extremities is adequate. Monitoring capillary refill time is another way to assess the adequacy of blood flow. To do this, the practitioner presses the patient's fingernail or toenail until it pales and then observes how long it takes to regain color once the pressure is released. Greater than average capillary refill time may be a sign of peripheral vascular disease or blocked arteries.

Cardiovascular System

The examination of the cardiovascular system focuses on the rate and rhythm of radial and carotid artery pulses (located at the wrist and neck), blood pressure, and the sounds associated with blood flow through the carotid arteries and the heart. After measuring and recording the rate and rhythm of radial and carotid pulses, the practitioner may listen with a stethoscope for abnormal sounds in the carotid arteries. Rushing sounds, called "bruits," may indicate narrowing of the arteries and an increased risk for stroke.

Examination also entails assessment of jugular vein pressure and listening (auscultation) with a stethoscope to heart sounds. Heart murmurs, clicks, and extra sounds are abnormal heart sounds associated with the functioning of heart valves. Some murmurs are considered "innocent" (normal variations), whereas others are indicators of serious malfunctioning of heart valves.

Abdominal Examination

Inspection of the abdomen focuses on the shape and movement of the abdomen and the presence of scars, lesions, rashes, and hernias (protrusion of an organ through a wall that usually encloses it). Using a stethoscope, the practitioner listens to the arteries that supply blood to the kidneys, listens to the aorta (the main artery that supplies blood to all the organs except the lungs), and listens for bowel sounds.

Percussion of the abdomen that produces a dull sound may indicate an abnormality, such as an abdominal mass. Percussion also is used to determine the size of the liver (the largest gland in the body, which produces bile to aid in the digestion of fats) that measures 6–12 cm in a healthy adult. An expanse of dullness around the liver or spleen (an organ on the left side of the body, below the diaphragm, that filters and stores blood) may indicate that these organs are enlarged.

A healthy liver is not tender or painful; palpation of the liver is used to check for pain or tenderness. The aorta may be palpated in the midline of the abdomen, and its pulse may be felt. The spleen is not usually palpable in healthy adults.

Breast and Pelvic Examination

Visual inspection of the breast focuses on symmetry, dimpling, swelling or discoloration of skin, and position of the nipple. Manual breast examination is performed by slowly and methodically palpating breast tissue in overlapping vertical strips using small circular movements from the midline to the axilla (armpit). The practitioner presses the nipple to observe whether there is any discharge (fluid) and also palpates the axilla for the presence of lymph nodes.

Annual (or more frequent) manual examination of the breast by a health practitioner supplements, but does not replace, monthly breast self-examination and regular mammography (breast x-rays able to detect tumors too small to be felt during manual examination) as recommended by a physician or other health care practitioner.

Pelvic examination often is performed after the breast examination, during a woman's physical examination by a primary care physician (doctor specializing in general practice, family practice, or internal medicine), an obstetrician–gynecologist (a physician specializing in women's reproductive health), a nurse-midwife, or nurse practitioner (also known as advance practice nurses, who have additional professional training, expertise, and certification).

With the patient lying down with her heels in stirrups and knees apart, the health care practitioner inspects the genitalia for redness, swelling, and infection. Using an instrument called a speculum, which is inserted into the vagina, the practitioner is able to view the cervix (opening of the uterus) and vaginal walls. At this time, a sample of tissue usually is obtained for a Papanicolaou (Pap) smear, which is examined microscopically for cervical cancer cells in the cytology laboratory.

Pelvic examination also involves manual palpation of the cervix to assess its shape, size, and mobility and to examine the uterus and ovaries. A rectal examination, during which the practitioner inserts a gloved finger into the rectum to check for polyps (protruding growths), also may be performed during this examination.

Neurologic and Mental Status Examinations

Neurologic examination considers mental status, cranial nerves (the twelve cranial nerves are olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial, acoustic, glossopharyngeal, vagus, accessory, and hypoglossal), muscle strength, coordination and gait, reflexes, and the senses. When a comprehensive neurologic examination is performed independent of a complete physical examination, it includes an eye examination to test visual acuity, visual fields, the reaction of the pupils to light, and extraocular muscles. However, when neurologic evaluation is performed as part of a complete physical examination, the eye examination usually has been performed prior to the neurologic examination.

Generally, the cranial nerves are assessed by observation as the health care practitioner asks the patient to demonstrate their use. For example, the facial nerve may be tested by watching patients open their mouths and clench their teeth. The practitioner also tests sensation to the parts of the face supplied by branches of the trigeminal nerve by applying sharp and dull objects to these areas and asking the patient to distinguish between them. Finally, the practitioner touches the patient's cornea lightly to observe whether the patient blinks—the corneal reflex is present if the patient blinks normally in response to the stimulation.

Evaluating the motor system involves assessment of muscle symmetry, tone, strength, gait, and coordination. Patients are observed performing different skills and walking. Reflexes also are tested and graded as "normal," "hypoactive," or "hyperactive." An example of reflex testing is when the practitioner strikes the patellar tendon just below the kneecap to observe contraction of the quadriceps muscle in the thigh and extension of the knee.

The sensory system is tested to determine whether there is loss of sensation in any body part. The practitioner may use light touch; the vibrations from a tuning fork; or hot, cold, or sharp objects to evaluate patients' abilities to perceive sensation accurately. The practitioner also may test discrimination—the ability to accurately interpret touch and position—by tracing a number on the patient's palm and asking the patient to name the number.

A preliminary evaluation of mental status aims to determine the patient's orientation, immediate and short-term memory, and ability to follow simple verbal and written commands. Patients are considered "oriented" if they can identify time, place, and person accurately. Immediate and short-term memories are tested when the practitioner poses simple questions for the patient to answer, and one's ability to follow commands is assessed by observing patients perform tasks in response to verbal or written instructions.

DIAGNOSTIC TESTING

Once the history and physical examination have been completed, the health care practitioner is often relatively certain about the cause of illness and the diagnosis. However, occasions occur when the history and physical examination point to more than one possible diagnosis. In such instances, the practitioner develops a "differential diagnosis"—a list of several likely diagnoses. The practitioner then may order specific diagnostic tests to narrow the list of possibilities. The results of these tests are evaluated in the context of the patient's history and physical examination.

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.