IgM anti-HBc levels were measured by the IMx Core-M Abbott assay in 939 serum samples in order to define a specific and sensitive cut-off value for diagnosis of chronic hepatitis B. The sera used were obtained from 52 chronic HBV patients and 10 HBV carriers with HCV or HDV co-infections and 155 asymptomatic subjects without evidence of liver disease. A Youden index value of 95.4% with 98% sensitivity and 97.4% specificity was obtained for an IMx Index value of 0.204 as cut-off. A one-year follow-up study with monthly tests has shown that quantitative analysis of IgM anti-HBc can serve as a noninvasive tool for monitoring HBV infection, and provides an accurate diagnosis of hepatitis B exacerbations. Significant elevations of IgM anti-HBc levels were associated with hepatitis B exacerbations in 96.2% of the cases but with none of the ALT flare-ups observed in HCV or HDV infected individuals. These results suggest that quantitative analysis of IgM anti-HBc provides the highest degree of confidence in definition of spontaneous and therapy-induced exacerbations or remissions of hepatitis B.