The earliest clinical sign of dental caries is a chalky white area on the tooth, which represents demineralization of the enamel. This white spot lesion occurs before the enamel surface breaks down and cavitation develops. It indicates subsurface mineral loss while the surface remains intact, making it a reversible stage if proper remineralization therapy is applied. Recognizing this non-cavitated lesion is crucial for early intervention to prevent progression to cavity formation, discoloration, or tooth mobility. Thus, the chalky white spot is the primary clinical indicator of initial caries activity.

Reference: Principles of Operative Dentistry, Stanely F. Malamed, 13th Edition.