Retrospective validation of a surveillance system for unexplained illness and death: New Haven County, Connecticut

Objectives. This study investigated retrospective validation of a prospecti ve surveillance system for unexplained illness and death due to possibly in fectious causes. Methods. A computerized search of hospital discharge data identified patien ts with potential unexplained illness and death due to possibly infectious causes. Medical records for such patients were reviewed for satisfaction of study criteria. Cases identified retrospectively were combined with prospe ctively identified cases to form a reference population against which sensi tivity could be measured. Results. Retrospective validation was 41% sensitive, whereas prospective su rveillance was 73% sensitive. The annual incidence of unexplained illness a nd death due to possibly infectious causes during 1995 and 1996 in the stud y county was conservatively estimated to range from 2.7 to 6.2 per 100 000 residents aged 1 to 49 years. Conclusions. Active prospective surveillance for unexplained illness and de ath due to possibly infectious causes is more sensitive than retrospective surveillance conducted through a published list of indicator codes, However , retrospective surveillance can be a feasible and much less labor-intensiv e alternative to active prospective surveillance when the latter is not pos sible or desired.