Detecting glaucoma with frequency-doubling technology perimetry : a comparison between N-30 and C-20 screening programs

Purpose: To evaluate the ability of frequency-doubling technology (FDT) perimetry in detecting glaucoma with N-30 and C-20 screening programs. Methods: Eighty eyes of 80 patients were enrolled (40 glaucomatous, 40 controls). Humphrey achromatic perimetry (standard automated perimetry, SAP) was considered as the "gold standard" for diagnosis. To assess whether N-30 screening program could detect more initial glaucomatous defects than C-20, glaucomatous patients included 20 cases with nasal step at SAP (pre-selected by medical chart inspection). Patients underwent two SAP examinations to confirm diagnosis; then two N-30 and two C-20 screening tests with frequency-doubling technology were performed in a randomized sequence. Finally, a frequency-doubling technology N-30 full-threshold examination was performed. Several criteria to define abnormality at frequency-doubling technology screening programs were evaluated. Results: For both C-20 and N-30 screening programs, the best parameter to detect glaucoma was the presence of at least 1 point with P < 5% (sensitivity = 87.5% for both tests and specificity of 90% and 95% for C-20 and N-30, respectively). Both screening procedures obtained a lower sensitivity (75%) in patients with a nasal step, whereas frequency-doubling technology full-threshold program was able to detect the initial defects in all cases. Conclusions: N-30 and C-20 screening procedures obtained similar results in well-defined glaucoma patients in terms of sensitivity and specificity. In the presence of a standard automated perimetry nasal step, diagnostic ability with both frequency-doubling technology screening strategies decreased and one quarter of nasal steps went undetected.