The aim of this study was to evaluate the impact of image modality and observer characteristics on radiographic caries diagnosis with particular emphasis on the false positive ( FP) test outcome. Eighty human teeth were radiographed by eight digital modalities and film. Eight observers scored caries lesions in each approximal tooth surface. Their use of image enhancement and time spent in assessing an image was recorded. Microscopy served as validation for the presence/absence of a lesion. Of 160 approximal surfaces, 63% were sound and 37% had lesions. Multivariate logistic regression analysis was performed with the FP test outcome as the dependent variable and the radiographic modality and observer's gender, experience, employment, image enhancement, and time use as the independent variables. The significant risk factors for an FP test outcome were: ( 1) gender: males scored fewer FP than females ( OR 0.26, Cl 0.18-0.36); ( 2) experience: observers with no experience with digital radiography had a six times higher risk for an FP ( OR 6.09, Cl 4.9-8.1); ( 3) radiographic modality: two of the digital systems had a significantly lower risk for an FP [ a phosphor plate system ( OR 0.55, Cl 0.37-0.73) and a sensor-based system ( OR 0.65, Cl 0.46-0.93)] and one phosphor plate system had a higher risk ( OR 1.57, Cl 1.17-2.13) than film; ( 4) time use: more time was spent when scoring an FP ( OR 1.05, Cl 1.03-1.05) than for any other test outcome. Type of employment and the use of image enhancement had no significant impact on the risk for an FP diagnosis. Copyright (c) 2007 S. Karger AG, Basel413170176