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    A variety of measures are employed in epidemiology, each of which has a specific definition and use.
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    When characterizing the likelihood of developing a disease within a specified period of time, the appropriate measure is risk.
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    Prevalence is used to describe the proportion of a population that is affected by a disease.
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    When measuring the rate of new occurrences of a disease, incidence is the appropriate measure.
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    Case fatality is used to describe the natural history of a disease and corresponds to the proportion of affected persons who die from that illness. Conversely, survival is the likelihood of escaping death from that illness.

image A 60-year-old previously healthy female research chemist recently developed shortness of breath and nosebleeds. On physical examination, the patient was pale and her pulse was elevated at 110 beats per minute. Her hematocrit was 20% (low), indicating anemia, her white blood cell count was 20,000/mL (elevated), her platelet count was 15,000/mL (low), and examination of her peripheral blood smear revealed atypical myeloblasts. The patient was hospitalized for suspected acute myelogenous leukemia. The diagnosis was confirmed by examination of a bone marrow aspirate and biopsy. Chemotherapy was started and about 3 weeks later, the patient’s temperature rose abruptly to 39°C, and her neutrophil count dropped to 100/mL (abnormally low). Although no source of infection was apparent, cultures were obtained of her blood and urine, and antibiotics were administered to cover a wide range of potential infections. These cultures confirmed the presence of Staphylococcus aureus in the blood.

Acute myelogenous leukemia (AML), also known as acute nonlymphocytic leukemia, is a heterogeneous group of disorders involving uncontrolled proliferation of primitive blood-forming cells. AML accounts for almost one third of all leukemias, with over 9000 patients newly diagnosed in the United States each year. This disease tends to occur in later life, with a median age at onset of 65 years. Males are at a slightly higher risk than females.

Although for most patients the cause of AML is unknown, a number of risk factors have been identified, including exposure to ionizing radiation, benzene, certain drugs, and perhaps cigarette smoke. This disease also occurs with unusual frequency among patients with certain congenital disorders—such as Down syndrome.

Patients with AML may present with a variety of symptoms, including weakness, fatigue, unexplained weight loss, infection, and bleeding. On physical examination, these patients often are pale, have multiple bruises, and have fevers, with evidence of localized infections. In some instances, enlargement of the lymph nodes, spleen, or liver may be found. Examination of blood specimens reveals anemia, low platelet counts, and markedly elevated leukocyte counts, with immature granulocytes abnormally appearing in the circulating blood. The bone marrow of these patients tends to be packed densely with cells, including a high proportion of immature cells.

The clinical management of AML involves an attempt to induce remission with chemotherapy. The likelihood of achieving remission ...

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