PURPOSE: To evaluate the impact of different definitions of primary open angle glaucoma (POAG) on assessment of the diagnostic validity of HeidelbergRetinal Tomography (HRT). DESIGN: Retrospective, cross-sectional study. METHODS: A search of medline (1992-2003) led to the retrieval of 181 papers containing definitions of POAG, including the eight visual field (VF)-based definitions used for this study. A sample of 193 normal subjects, 222 patients with suspected POAG, and 103 with POAG underwent HRT and the 24 IIHumphreyVF examinations to assess the diagnostic validity of HRT. POAG was defined on the basis of Glaucoma Hemifield Test (GHT) “out of normal limits”associated with corrected pattern standard deviation (CPSD) > 2 dB and intraocular pressure (IOP) < 22 mm Hg. The VFs were re-analyzed and categorized according to the other eight definitions of POAG: IOP formed part of all the definitions, whereas the appearance of the optic disk did not. The sensitivity and specificity of each scenario were calculated by standard procedures. RESULTS: The definitions of POAG found in the literature included 17 IOP criteria, more than 15 optic disk criteria, and more than 30 VF criteria. The sensitivity of HRT ranged from 0.51 to 0.80, and its specificity from 0.94 and 0.95 when the patients with suspected POAG were excluded; diagnostic validity was much lower and still variable when the POAG suspects were included with the normal or the POAG groups. CONCLUSIONS: The most commonly used VF-based definitions of POAG led to substantial differences in the sensitivity and specificity of HRT when using the same large sample of normal subjects and POAGpatients. Astandard definition of POAG is needed to make diagnostic investigations more accurate and comparable. PURPOSE: To evaluate the impact of different definitions of primary open angle glaucoma (POAG) on assessment of the diagnostic validity of Heidelberg Retinal Tomography (HRT). DESIGN: Retrospective, cross-sectional study. METHODS: A search of medline (1992-2003) A sample of 193 normal subjects, 222 patients with suspected POAG, and 103 with POAG underwent HRT and the 24 IIHumphreyVF examinations to assess the diagnostic validity of HRT. POAG was defined on the basis of Glaucoma Hemifield Test (GHT) “out of normal limits” associated with corrected pattern standard deviation (CPSD)> 2 dB and intraocular pressure (IOP) <22 mm Hg. The VFs were re-analyzed and categorized according to the other eight definitions of POAG: IOP formed part of all the definitions, but the appearance of the optic disk did not. The sensitivity and specificity of e ach scenario were calculated by standard procedures. RESULTS: The definitions of POAG found in the literature included 17 IOP criteria, more than 15 optic disk criteria, and more than 30 VF criteria. The sensitivity of HRT ranged from 0.51 to 0.80, and its specificity from 0.94 and 0.95 when the patients with suspected POAG were excluded; diagnostic validity was much lower and still variable when the POAG suspects were included with the normal or the POAG groups. CONCLUSIONS: The most commonly used VF-based definitions of POAG led to substantial differences in the sensitivity and specificity of HRT when using the same large sample of normal subjects and POAG patients. Astandard definition of POAG is needed to make diagnostic investigations more accurate and comparable.