SAP measurements easily swayed by various factors

The authors sought to determine the influence of several factors on standard automated perimetry (SAP) results in glaucoma patients. The most important factor found in this study is that even low levels of false-positive responses can significantly affect the mean deviation (MD). They also found that technician experience, time of day and season have a significant influence on the MD of SAP. While it is well understood that patient factors, such as level of alertness, influence patient performance on automated visual field testing, this study illustrates some of the other factors that may affect visual field testing.

The authors used prospective longitudinal Humphrey field analyzer 30-2 Swedish interactive threshold algorithm data from 160 glaucoma patients (160 eyes). Mean follow-up was 9.4 years, with on average 10.8 tests per patient. Approximately 0.2 dB lower MD values were found for inexperienced technicians (P < 0.001), tests performed after lunch (P < 0.001), and tests performed during the summer or fall (P < 0.001). The effects of time of day and season appeared to depend on disease stage.

Independent of these effects, the percentage of false-positive answers strongly influenced the MD, with a 1 dB increase in MD per 10 percent increase in false-positive answers.

The authors advise clinicians to be cautious in interpreting visual fields that have greater than 5 percent false positives, even though the visual field instrument only flags a rate above 15 percent.

In addition, while the effect of time of day on MD was small, its influence on pointwise analyses was not performed, and the authors recommend testing at the same time of day. Seasonal influence on visual field performance is consistent with prior studies indicating increased retinal sensitivity during seasons with a shorter period of daylight, although a direct link is still speculative.

They also conclude that compared with the MD, the pattern standard deviation appeared to be less influenced by the studied factors and might thus be the preferred global index in early glaucoma.