What are the three types of case definitions which help in identifying specific cases?
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CDC cuts US SARS Case Count in Half During 2003, there was a worldwide epidemic of SARS. The news release below is titled “CDC cuts US SARS case count in half”. How was CDC able to cut the rates for this disease in half from one reporting period to the next? Read the news release.... July 17, 2003 (CIDRAP News) – Because of a change in the case definition for SARS (severe acute respiratory syndrome), the United States has had only half as many suspected and probable cases of the illness as previously reported, federal health officials said today. The Centers for Disease Control and Prevention (CDC) said the total case count is now 211 instead of 418, a 49.5% reduction. The official tally now is 175 suspected and 36 probable cases, down from 344 suspected and 74 probable cases. The change is a result of excluding all cases in which convalescent blood samples—those collected more than 21 days after illness onset — tested negative for the SARS coronavirus, the CDC said in a news release. "Exclusion of these cases with negative convalescent serum provides a more accurate accounting of the epidemic in the U.S.," the agency said. The Council of State and Territorial Epidemiologists recommended changing the SARS case definition to exclude cases with negative convalescent serum tests. The recommendation is based on evidence that 95% of SARS patients mount a detectable antibody response in the convalescent phase, the CDC said. The revised case definition and case count are detailed in the Jul 18 issue of Morbidity and Mortality Weekly Report, published online today. "Serologic testing results suggest that a small proportion of persons who had illness consistent with the clinical and epidemiologic criteria for a U.S. case of suspect or probable SARS actually had SARS," the article states. "The case definition captures an array of respiratory illnesses that cannot be easily distinguished from SARS until laboratory testing results for SARS and other agents are performed." The sensitive case definition allowed for rapid investigation of possible cases and public health steps to prevent spread of the disease, the report adds. So what happened? The CDC changed the case definition once lab results were available. With the new definition, the 21 day serum had to test positive for the SARS coronavirus or it was no longer considered a SARS case. 418 cases was diminished to 211 cases just by changing the case definition. The case definition for a disease has substantial impact. Another example of a change in case definition occurred in the early 1990s when HIV was beginning to be understood. Epidemiological studies found that cervical cancer cases were resulting from infection with HIV. So, a revision resulted in including cervical cancer cases in the HIV count. As you proceed through this course it will be important that you provide specific case definitions for diseases for which you are conducting an epidemiologic investigation.. You may use a medical reference or ICD codes that classify diseases. The ICD (International Classification of Disease), is an international organization that meets to assign three or four digit codes to every possible cause of death. These are very specific. We are currently using ICD revision 10. Physicians or medical investigators may belong to professional organizations or societies that define cases somewhat differently than the CDC. It is important to be explicit about the case definition used in a study and to what else it is comparable. Below are further sources of case definitions that may be helpful to you in epidemiologic investigations for this course. What are the types of case definition?Case definition categories
Confirmed, probable and suspect case categories are commonly used, although different terminology may be found (e.g., “persons under investigation”).
What is a specific case definition?A case definition is a set of standard criteria for classifying whether a person has a particular disease, syndrome, or other health condition. Some case definitions, particularly those used for national surveillance, have been developed and adopted as national standards that ensure comparability.
What are the four components of case definition?A case definition includes criteria for person, place, time, and clinical features. These should be specific to the outbreak under investigation. "Person" describes key characteristics the patients share in common.
Why is it important to know case definition?The use of case definitions is very important in epidemiology in order to standardize criteria for identification of cases. All case definition must include the three classical dimensions of epidemiological variables: time, place and person.
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