To quantify the risk of severe anaphylaxis due to drugs and other exposures in hospital patients, a study of incident cases has been in progress since 1992, with data collection in Hungary, Spain, India, and Sweden. All cases of anaphylaxis that develop after admission to participating hospitals are enrolled. To confirm the diagnosis, clinical information for potential cases is reviewed by two physicians without knowledge of exposures, according to an algorithm developed for the study. Confirmed cases are classified as definite, probable, or possible anaphylaxis. As of March 1995, 123 cases were enrolled in Budapest, Barcelona, and Bombay/Pune (the study began in January 1996 in Sweden): 99 were classified as definite or probable anaphylaxis, and 24 as possible. Two of the 123 cases were fatal (2%). Based on the definite and probable cases and a denominator of 481,752 individuals in the nine participating institutions for which the age and sex distribution was known, we estimated the overall risk of severe anaphylaxis to be 154 per million hospital admissions; when the possible cases were included, the risk was 196 per million. The risk was higher among women, and it varied, although not linearly, according to age. A major problem in the epidemiologic evaluation of anaphylaxis has been resolved in the present study with the development of a case definition that, contrary to usual clinical practice, is independent of exposure. The results thus far indicate that severe anaphylaxis occurs infrequently among hospitalized patients and is rarely fatal.
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