A variety of measures are employed in epidemiology, each
of which has a specific definition and use.
When characterizing the likelihood of developing a disease
within a specified period of time, the appropriate measure is risk.
- Prevalence is used to describe the proportion of a population
that is affected by a disease.
When measuring the rate of new occurrences of a disease, incidence
is the appropriate measure.
- 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.
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
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 ...