In our study, FDT detected more eyes with glaucomatous optic neuropathy as having visual defects than did SAP-FT. The two tests agreed moderately well because those deemed abnormal by SAP-FT were, for the most part, a subset of those deemed abnormal by FDT. The global indices of the two tests correlated moderately well. To put the present results into perspective, our analyses with SAP-FT as the standard of comparison support the literature, which shows the FDT finds defects earlier than SAP-FT.
14 17 20 26 33 34
Previous studies have shown a strong correlation between FT and SITA MD and PSD.
8 Test–retest variability of FT and SITA tests are similar in patients with glaucoma
2 and in normal control subjects.
1 One study found differences in average MD between the threshold strategies, but these differences are probably not clinically significant.
4 Another study did not find any differences.
35 Heijl et al.
35 have demonstrated that it is generally safe to switch strategies, although it is preferable to establish new baselines.
Some differences have been reported, however. The average light sensitivity is approximately 1 dB better for SITA.
4 9 The normal confidence bands are slightly tighter, and the normal decrease in visual sensitivity with age may be smaller with SITA.
3 Finally, more total and PD points were abnormal with SITA in one study, but in not another.
4 35 The size and severity (by both the HAP criteria
31 and AGIS (Advanced Glaucoma Intervention Study) scores
36 ) of glaucomatous defects with SITA are similar to SAP-FT, but defects are significantly shallower on SAP-SITA.
6
When SAP-SITA was used as the standard of comparison, FDT found a similar number of eyes abnormal. However, the agreement was poor between SAP-SITA and FDT, in that different eyes were deemed normal and different ones abnormal by the two tests. That is, these two tests did not agree, as well, on which eyes had field loss. SAP-SITA and FDT global indices correlated moderately well.
There are several explanations for the differences found. First, the SITA threshold algorithm could be more sensitive to early visual field loss than SAP-FT. The properties of the algorithm (e.g., tighter confidence intervals) may have increased detection in some eyes. This would explain the higher number of eyes identified with glaucomatous optic neuropathy, but it does not explain why the eyes identified differed from those found by FDT. Also, SITA uses prior knowledge in determining thresholds, which is not used by SAP-FT or by the MOBS algorithm of the FDT N-30. This knowledge used by SITA is based on models of normal and glaucomatous visual fields, relying on information about age-corrected normal values at each test point, frequency-of-seeing curves, and correlations between thresholds at different test locations.
12 These assumptions may influence threshold results at some values, possibly introducing differences between SAP-SITA and SAP-FT and could also affect comparison with FDT. Second, SAP-SITA could show more false positives. However, a similar percentage of eyes were identified as abnormal using the SITA and FT algorithms in the post hoc analysis of healthy eyes, indicating that the criteria for abnormality were comparable between the two tests. This suggests that an elevated number of false positives by the SITA algorithm is not a likely explanation, although our sample for the post hoc analysis was small (
n = 39). Finally, SAP-SITA could detect different types of patients than FDT. FDT and SAP measure different visual functions. SAP is nonspecific, in that most retinal ganglion cell (RGC) subtypes can respond to the stimulus. FDT is a visual-function–specific test, in that the low-spatial-frequency, high-temporal-frequency stimulus is thought to target a subset of RGCs, the magnocellular cells.
21 25 27 In a previous study comparing SAP with a variety of visual-function–specific tests, including FDT, Sample et al.
26 found individual differences in which perimetric test detected field loss earliest. Longitudinal data were not available for all participants in this study, but will be helpful in answering these questions when they become available.
The value of a new technology often relies on comparison with a standard. For instance, FDT is thought to detect more eyes with visual field loss in early glaucoma than the standard against which it has been most often compared—SAP-FT. If the standard changes, then the value of the new technology might be interpreted slightly differently. The present results support the utility of FDT, but perhaps in a different light when the evaluation is made with SAP-SITA. Assuming the present results accurately reflect the relationship of FDT and SAP-SITA, then FDT’s major contribution may be as a complement to SAP-SITA for detecting visual loss in eyes missed by SAP-SITA.
Updating the standard of comparison when new algorithms and/or software become available is unavoidable. Newer studies are likely to use the SAP-SITA algorithm as the reference, and it is not sufficient to know how the two standards differ from one another (thorough comparisons of SAP-SITA and SAP-FT have been done
1 2 3 4 5 12 35 ). The differences should also be taken into consideration when making decisions about the value of other technologies and when making clinical decisions about patients who have a mix of new and old technologies during follow-up.
In summary, this study shows how shifting from SAP-FT to SAP-SITA as the standard of comparison changes the relationship to FDT results. Although correlational analyses were unaffected, SAP-FT tended to flag a subset of the eyes found abnormal by FDT, whereas SAP-SITA classified a different group of eyes as abnormal than did FDT, even though the number of eyes flagged by each was similar. This means that a patient with early visual field loss is less likely to show a visual field defect on SAP-FT than on either of the other two tests (by our criteria). SAP-FT tends to flag a visual field defect in a subset of the same eyes as FDT. In contrast, SAP-SITA may tend to identify different eyes. Thus, if a patient with early visual field loss has both FDT and SAP-SITA during follow-up, the results of the two tests may not concur. If the defects are repeatable, it could mean that more eyes will be identified by using a combination of the two tests, but a longitudinal study is needed to confirm this.