Purpose:
To determine the influence of several factors on standard automated perimetry test results in glaucoma.
Methods:
Longitudinal perimetry data (Humphrey Field Analyzer; 30-2; SITA fast) from 160 eyes of 160 glaucoma patients were used. The influence of test reliability, technician experience, time of day and season on the mean deviation (MD) was determined by performing linear regression analyses of MD against time on series of visual fields and subsequently performing a multiple linear regression analysis with the residuals as dependent variable and the factors mentioned above as independent variables. A resampling technique was used because residuals from a single subject are not independent. We performed these analyses both with and without adjustment for the reliability of the test result as depicted by the reliability indices (fixation losses and false positive and false negative answers). Patients were stratified according to baseline MD and age, with -6 dB and 65 years as cut-off points, respectively.
Results:
Mean follow-up was 9.4 years with on average 10.8 tests per patient. Technician experience, time of day, and season were all associated with the MD. Typically 0.5 dB higher MD values were found for experienced technicians (especially in older patients with early glaucoma; P<0.005), tests performed in the early morning (in early glaucoma; P<0.001) or later on in the morning (in moderate/severe glaucoma; P<0.005), and tests performed in the spring (in elderly patients with moderate/severe glaucoma; P<0.001). In addition, apart from differences in the mean effect size, the variability in MD residual was also greater in inexperienced technicians (see figure). Independent of these effects, the percentage of false-positive answers influenced the MD significantly (P<0.001) but that of the fixation losses and false-negative answers did not. Every 10% increase in false-positive answers increased the MD by 1 dB.
Conclusions:
The percentage of false-positive answers is the only reliability measure that should be taken into account. In addition, technician experience, time of day, and season may together cause MD inter-test differences exceeding 1 dB.
Keywords: perimetry • visual fields • clinical (human) or epidemiologic studies: systems/equipment/techniques