Zeiss Humphrey II-i series, User Manual

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Guided Progression Analysis (GPA)
Humphrey Field Analyzer II-
i
series User Manual 2660021145640 A
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were invented to calculate the expected variability ranges, and the resulting significance limits have
been independently validated.
In the example shown in Figure 8.5 , the arrow on the left indicates a point with the value -6 on the
Deviation from Baseline Plot that is not flagged (see right arrow). A neighboring point to the lower
right with a lesser deviation value (-4) is flagged. Knowledge of the complex patterns of test
variability is built into the GPA analysis.
Figure 8.5 Significance of Change Depends on Test Point Location
Out-of-Range Points
Some points in the GPA are not eligible for analysis. These are marked with an “X” on the
Progression Analysis Probability Plot and considered to be “out-of-range” points. In these cases,
GPA is unable to determine whether the change from Baseline is statistically significant. Most points
that fit these criteria are points that are quite depressed to begin with. Any change from the
depressed Baseline values may be indistinguishable from the amount of change that might be due
to the normal variability experienced in a visual field test. Often the “out of range” points are found
to be points that have reached threshold values that are at maximum brightness (< 0 dB).
Occasionally, points marked with an “X” are due to a selection of Baseline tests that are not truly
representative of the patient’s visual field status. Sometimes the points can be analyzed with a
change of Baseline tests. See the case examples that follow.
A Note of Caution
When considering the probability statements in this statistical package, it is important to be
conscious of their limitations. They are an aid to interpretation, not a diagnosis. The doctor’s
judgment is still the most important element in determining the clinical significance of perimetric
findings.
You must always apply clinical judgment when using these tools. This software represents an attempt
to aid the practitioner in making medical decisions. There will be situations where it will not give the
proper analysis, either due to its own limitations or because it was applied to inappropriate data.
Obviously, the practitioner must bear the ultimate responsibility for all decisions and diagnoses. In
cases of uncertainty, consultation with sub-specialists is often the prudent course of action.