State health department response to disease cluster reports: a protocol for investigation

State health departments are increasingly faced with the task of responding to reports of apparent cancer and other disease clusters. Since 1979, 141 requests for investigation have been received by the Wisconsin Division of Health, with over 60% of these requests occurring since 1985. Mounting public concern and limited resources have resulted in the development of a "disease cluster investigation and analysis protocol." Protocol steps include: 1) circumscribing the cluster; 2) ascertaining cases; 3) assessing risk of the exposed versus a referent population; 4) statistically analyzing disease rates; 5) examining potential exposure; 6) assessing biologic plausibility; 7) determining cluster significance and need for further investigation; and 8) reporting results. To demonstrate the protocol, the authors present a case example of an investigation of an apparent cancer cluster. Since 1979, 62 reports were resolved with initial contact with informant education (step 1), 61 reports required descriptive analysis (steps 1-8) with no site visit, and 18 reports required site visits. None of the reports required further in-depth epidemiologic investigation. This protocol provides a systematic approach to investigation and analysis, prioritizes the need for more in-depth study, and, when necessary, assuages community concerns when a disease cluster is reported.