Short-term fluctuation as an estimate of variability in visual field data

The short-term fluctuation index (SF) is one of several values that provide an indication of a patient's response reliability during an automated perimetry examination. The authors investigated the number of visual field locations used and the number of determinations per location as factors affecting the SF estimate. A computer simulation program for perimetry was used to measure the SF index for 350 normal visual fields with various levels of response fluctuation. As expected, the variability of the SF estimate decreased as the number of locations used to estimate SF increased. There was a more important finding that, for an equal number of threshold estimates, a larger number of determinations at a smaller number of locations produced greater consistency in the SF estimate (eg, ten determinations at two locations instead of two determinations at ten locations). However, it is also important to sample from a representative spatial distribution of visual field locations. These results suggest that five determinations at four locations in the visual field is optimal for most clinical perimetric testing situations.