Objectives: Subgingival dental calculus is recognized as an important contributing risk factor in the initiation and perpetuation of inflammatory forms of human periodontitis, and its removal from diseased tooth root surfaces constitutes a critical clinical end-point in periodontal therapy. As a result, the reliable detection of subgingival dental calculus on tooth root surfaces is of paramount importance in periodontal diagnostics. A wide range of manual dental instruments have been employed to detect by tactile sensation the presence of subgingival dental calculus deposits on teeth. However, the Old Dominion University (ODU) 11/12 dental explorer is presently used by all regional dental licensure examining boards in the United States to identify subgingival dental calculus on patients. Interestingly, little data is presently available to validate the ability of the ODU 11/12 dental explorer to reliably discriminate between dental calculus- positive and –negative tooth root surfaces. As a result, the purpose of this study was to to assess, with an in vitro typodont model system, the ability of an ODU 11/12 dental explorer to accurately identify subgingival dental calculus on tooth root surfaces. Methods: A total of 108 subgingival sites on mandibular posterior plastic teeth, of which 73 (67.6%) exhibited artificial dental calculus deposits, were mounted within on typodont models of the human oral cavity, comprised of white plastic teeth emerging from and surrounded by anatomically-accurate pink silicone gingival and palatal soft tissues. Each typodont was attached to a phantom head with simulated soft tissue mouth shrouds. Sheep blood was irrigated into subgingival and interproximal areas around ii typodont teeth to simulate gingival tissue inflammation, and artificial saliva applied onto supragingival typodont tooth surfaces to further simulate typical oral cavity conditions in humans. The 108 test subgingival surfaces were then evaluated for subgingival dental calculus with an ODU 11/12 dental explorer in duplicate by a primary examiner, who was a board-certified periodontist with 30 years of clinical specialty experience. A periodontist initially educated as a dental hygienist and possessing 45 years of combined dental hygiene-periodontics clinical experience, scored all of the test subgingival tooth surfaces once as a secondary examiner. The diagnostic performance of the ODU 11/12 dental explorer, relative to in vitro detection of subgingival dental calculus, was assessed among all test root surfaces, as well as among proximal and non-proximal root surfaces, with calculations of sensitivity, specificity, positive predictive value, negative predictive value, positive likelihood value, negative likelihood value, diagnostic odds ratio, accuracy (diagnostic effectiveness), and Youden’s Index. Results: As utilized by the primary examiner, the ODU 11/12 dental explorer yielded a sensitivity of 91.2%, specificity of 78.4%, positive predictive value of 82.5%, negative predictive value of 88.9%, positive likelihood value of 4.2, negative likelihood value of 0.1, diagnostic odds ratio of 38.5, accuracy (diagnostic effectiveness) of 85.2%, and Youden’s index value of 0.70, for in vitro detection of subgingival dental calculus. Similar diagnostic test findings for the ODU 11/12 dental explorer were found on proximal (mesial and distal) and non-proximal (buccal and lingual) root surfaces, with accuracy (diagnostic effectiveness) values attained of 82.4% and 84.3%, respectively. Good reproducibility (kappa = 0.62) was found in duplicate scoring by the primary examiner of subgingival dental calculus with the ODU 11/12 dental explorer. The iii secondary examiner produced even better performance outcomes with the ODU 11/12 dental explorer, providing a sensitivity of 91.2%, specificity of 86.3%, positive predictive value of 88.1%, negative predictive value of 89.8%, positive likelihood value of 6.7, negative likelihood value of 0.1, diagnostic odds ratio of 66.5, accuracy (diagnostic effectiveness) of 88.9%, and Youden’s index value of 0.78, for in vitro detection of subgingival dental calculus. Similar to the primary examiner, the secondary examiner also found relatively close agreement in diagnostic test findings for ODU 11/12 dental explorer on both proximal and non-proximal root surfaces, with accuracy (diagnostic effectiveness) values attained of 85.2% and 92.6%, respectively. A good level of agreement (kappa = 0.62) was found between the primary and secondary examiners in their in vitro scoring of subgingival dental calculus with the ODU 11/12 dental explorer. Conclusions: These study findings provide important in vitro validation for continued use of an ODU 11/12 dental explorer for detection of subgingival dental calculus on dental licensure examining board examinations. The ODU 11/12 dental explorer exhibited a high level of in vitro discrimination between subgingival dental calculus-positive and calculus-negative tooth root surfaces in a typodont model system with experienced periodontist examiners. The ODU 11/12 dental explorer performed with a similar high level of diagnostic accuracy on both proximal and non-proximal tooth root surfaces, and exhibited good reproducibility in duplicate assessments made by the primary examiner, and showed good agreement between evaluations made by the primary and secondary examiners. Based on these in vitro findings, routine clinical utilization of the ODU 11/12 dental explorer in dental practice for the detection of subgingival dental calculus is recommended. / Biology