Sensitivity and specificity of the GDx: clinical judgment of standard printouts versus the number

Thomas P Colen, Hans G Lemij

Research output: Contribution to journalArticleResearchpeer-review


PURPOSE: The Number is a standard parameter of the GDx that reportedly distinguishes normal and glaucomatous eyes. The authors evaluated the sensitivity and specificity of the Number and examined whether expert clinical judgment of GDx printouts leads to a better separation.

MATERIALS AND METHODS: Two experienced observers judged 800 GDx scans on 400 randomly presented printouts from 200 glaucoma patients and 200 age-matched normal subjects. The diagnosis was based on the symmetry analysis printout and was per patient rather than per eye. The observers assessed sensitivity for all glaucoma patients together, and separately for mild, moderate, and severe glaucoma. Their specificity was determined in the group of normal subjects. The same procedure was performed for the Number, at various critical values.

RESULTS: Both observers discriminated better than the Number. At a critical value of 23, the specificity of the Number was 81.5%, which matched the lowest specificity of the 2 observers: 82.5% and 92.0% for observers 1 and 2, respectively. At these specificities, the sensitivity of the 2 observers and of the Number were 92.0%, 89.5%, and 85.5%, respectively. The sensitivity increased with the severity of glaucoma. The Kappa values for intraobserver agreement were 0.80 and 1.0.

CONCLUSIONS: The Number yielded acceptable sensitivity and specificity values at a critical value of 23 in this test population. However, the clinical judgments of the printouts by both expert observers resulted in a better separation between normal and glaucomatous eyes, particularly in the group with mild glaucoma.

Original languageEnglish
Pages (from-to)129-33
Number of pages5
JournalJournal of Glaucoma
Issue number2
Publication statusPublished - Apr 2003


  • Adult
  • Aged
  • Aged, 80 and over
  • Birefringence
  • Diagnostic Techniques, Ophthalmological/instrumentation
  • Glaucoma/diagnosis
  • Humans
  • Lasers
  • Middle Aged
  • Nerve Fibers/pathology
  • Observer Variation
  • Optic Disk/pathology
  • Optic Nerve Diseases/diagnosis
  • Predictive Value of Tests
  • Reproducibility of Results
  • Retinal Ganglion Cells/pathology
  • Sensitivity and Specificity


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