Within-test variability of frequency-doubling perimetry using a 24-2 test pattern.

PURPOSE: To evaluate patient-response (within-test) variability for targets of the smaller frequency-doubling technology perimetry test that employs a 24-2 stimulus-presentation pattern.

METHODS

Patient-response variability was examined using the method of constant stimuli for standard (10 degrees ) and small (4 degrees ) customized frequency-doubling technology perimetry stimuli presented on a CRT screen. Small stimuli were designed for use in a 24-2 test pattern. Matched test locations were examined in 24 subjects (8 normal, 8 in whom glaucoma was suspected, and 8 glaucoma patients). Threshold sensitivity (in decibels for the 50% detection level) and variability (interquartile range in decibels) were obtained from frequency-of-seeing curves derived from data fitting with cumulative Gaussian functions. Groups were compared using a two-way ANOVA.

RESULTS

Thresholds obtained using standard and small stimuli were highly correlated (R = 0.94, P < 0.001, Pearson correlation), although smaller targets systematically estimated sensitivity to be 2.0 dB (95% CI, 1.7-2.4 dB) lower than standard targets. No significant difference in patient-response variability was observed between standard and small targets (P = 0.067), although both target sizes demonstrated small but significant increases in variability with reduced sensitivity. Mean (SD) patient-response variability for the normal, suspect, and glaucoma groups was 1.0 (0.6), 0.9 (0.4), and 1.8 (1.4) dB for standard-sized stimuli and 1.1 (0.8), 1.5 (1.2), and 2.0 (0.9) dB for small stimuli.

CONCLUSIONS

Small (4 degrees ) frequency-doubling technology perimetry targets have variability characteristics that are not statistically significantly different from those observed for standard-sized (10 degrees ) stimuli. Reduction in frequency-doubling technology perimetry stimulus size necessary to produce 24-2 test resolution is unlikely to affect test repeatability. Smaller, more numerous stimuli may offer clinical advantages both in terms of detecting small defects and identifying progressive loss.