“Probable” Versus “Confirmed” Leptospirosis: An Epidemiologic and Clinical Comparison Utilizing a Surveillance Case Classification

PURPOSE: For surveillance purposes, the Centers for Disease Control and Prevention and Council of State and Territorial Epidemiologists (CDC/CSTE) have defined two case classifications for leptospirosis: “confirmed” and “probable.” The objective of this study was to provide data to refine the current surveillance case classifications. METHODS: All reported leptospirosis infections from exposures within the State of Hawaii, 1974 to 1998 meeting CDC/CSTE “confirmed” and “probable” case classifications were compared on a number of clinical and epidemiologic parameters. RESULTS: Confirmed cases (n = 276) had more severe clinical manifestations than probable cases (n = 180); however, probable cases with higher peak microscopic agglutination test (MAT) titers (⩾1:800) were clinically and epidemiologically comparable to confirmed cases. In addition, 77 cases demonstrating fourfold or greater MAT titer increases in paired serum collected less than two weeks apart (currently excluded from the “confirmed” case classification) were also comparable to confirmed cases. CONCLUSIONS: Our findings support amending the current CDC/CSTE surveillance confirmed case classification to include demonstration of a fourfold or greater MAT titer increase in paired serum, irrespective of the interval between specimen collection. Consideration should also be given to including single MAT titer ⩾1:800 as a criterion for “confirmation.” These changes would both simplify and expedite the surveillance confirmation of leptospirosis.