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A case–control study is an observational study
in which subjects are sampled based upon presence or absence of
disease and then their prior exposure status is determined.
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Case–control studies are statistically efficient
and cost-effective for the study of rare diseases, and multiple
risk factors can be investigated in a case–control study.
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Newly diagnosed persons with disease are referred to as incident
cases, whereas previously existing cases are referred to as prevalent
cases.
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Ideally, the controls should have a prevalence of exposure
that is the same as the population of unaffected persons.
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A population-based study is one in which cases and controls
are sampled from a defined population, such as a metropolitan area.
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A hospital-based sample of cases and controls may be convenient
and inexpensive to collect, but may be biased by factors that affect
the likelihood of hospitalization for cases and controls.
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If sampling of cases, controls, or both is influenced by prior
exposure history, then a selection bias may be present.
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Confounding occurs when the apparent effect of the exposure
of interest is attributable in whole or in part to some other factor.
- Matching in a case–control study involves sampling
of controls to parallel selected characteristics of cases in order
to reduce the likelihood of confounding by the matched features.
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The odds ratio is a measure of association between the exposure
and disease that can be calculated in case–control studies.
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A 55-year-old woman was in excellent health until 2 weeks before
admission, when she developed malaise, low-grade fever, cough, and
generalized muscle pain. Although she took aspirin, her symptoms
became worse over the next several days, in particular increased
muscle pain, which made it very difficult for her to rise from a
chair. She then consulted her personal physician, who performed
a thorough evaluation. The patient’s history was unremarkable
except for insomnia over the previous year, which she treated with
self-prescribed
l-tryptophan. On physical
examination, she had mild, diffuse muscle tenderness and a mild, erythematous
maculopapular rash over much of her body. Laboratory examination
was remarkable for elevations of her blood eosinophil count (2000
cells per mm
3, < 250 cells per mm
3) and mildly
elevated aldolase levels. Eosinophilia-myalgia syndrome (EMS) was
diagnosed.
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In November 1989, researchers from the Centers for Disease Control
and Prevention (CDC) and local health departments published the
first description of EMS. This newly recognized syndrome is characterized
by incapacitating myalgias (muscle pains), elevated eosinophil counts,
and in some patients, arthralgias (joint pains), skin thickening,
hair loss, and interstitial lung disease.
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EMS was first recognized in October 1989, when astute physicians
determined that three people with unexplained myalgias and eosinophilia
had consumed l-tryptophan, an essential
amino acid available without prescription in drug and health food
stores. Prompt response by health departments quickly led to case–control
studies, the results of which suggested that ingestion of l-tryptophan was the cause of EMS. l-Tryptophan–containing products ...