A total of 245 CIGTS patients attended study follow-up visits between January 2004 and January 2005, the time frame in which the VF conversion protocol was under way. A center-specific randomization schedule was used to determine the order of VF testing of the patient’s study eye. The FT test and the SITA-Standard test were then obtained in the specified order. Recovery time was allowed between testing, with the amount dependent on the patient’s assessment of readiness. The CIGTS protocol required reliable tests, based on scoring of fixation losses (criterion of <33%), false-positive and -negative errors (criterion of <33%), and the short-term fluctuation (SF) value (criterion, ≤4.0 dB). Because the SITA-Standard test does not produce an SF value, its reliability was based on the first three parameters. In two patients, pupil dilation was induced between the two VF tests, as indicated by the intertest pupil diameter being 4 mm disparate. These two patients’ data are not included in this report, yielding n = 243 with comparable VF test results.
Both VF test results were scored according to the CIGTS VF scoring algorithm,
1 11 which assigns weights to points on the VF test’s total deviation probability plot according to the extent of departure from normal values, as expressed by point-specific probabilities, which are empirically derived percentiles from the distributions of values at each of the 52 points from age-specific sets of normal subjects collected by the manufacturer.
2 The proprietary distributions are built into the VF test software and are not available for inspection. The probability at each of the 52 points is reported as no defect or
P ≤ 0.05, ≤ 0.02, ≤ 0.01, or ≤ 0.005, meaning that the measured value at that point was at or below the respective percentile of the age-specific empiric distribution at that position in the field for normal subjects. A point is called defective if its probability is 0.05 or less and it has at least two neighboring points with probabilities of 0.05 or less in the same vertical hemifield (superior or inferior). A weight is assigned depending on the minimum depth of the defect at the given point and the two most defective neighboring points. A minimum defect of 0.05, 0.02, 0.01, or 0.005 is given a weight of 1, 2, 3, or 4, respectively. A point without two neighboring points all depressed to at least
P ≤ 0.05 is given a weight of 0. For example, a point at
P ≤ 0.01 with only two neighboring points of defect, both at
P ≤ 0.05, would receive a weight of 1. The weights for all 52 points in the field are summed, resulting in a value between 0 and 208 (52 × 4). The sum is then scaled to a range of 0 to 20 (by dividing by 10.4), resulting in a score that is a nearly continuous measure of VF loss. Other Humphrey VF test parameters that are common to both testing procedures—test duration, pupil diameter, mean deviation (MD), pattern SD (PSD), and Glaucoma Hemifield Test (GHT) result
12 —were recorded.
Comparisons between test results were made with paired Student’s
t-tests and scatterplots for continuous variables. For categorical variables, we used the McNemar test for dichotomous variables and the Bowker test for symmetry
13 for more than two categories. Factors predictive of differences in test results (SITA minus FT) were evaluated by linear regression. Data analyses were performed on computer (SAS, ver. 9.1; SAS, Cary, NC).
14
This research adhered to the tenets of the Declaration of Helsinki. All CIGTS patients gave written informed consent to participate, and the institutional review boards at the CIGTS clinical centers approved the study.