Zeiss HUMPHREY 720i, Руководство пользователя

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6-4 TEST RELIABILITY
REV. B 02/03 PN 51680-1 HUMPHREY FIELD ANALYZER II -
i
series
Even with the most careful perimetric technique, sometimes test results are unreliable. To
assist with evaluating reliability, the HFA II offers several tools that measure accuracy and
consistency. “Catch trials” are special stimuli (or lack of) which are used for monitoring.
Fixation losses When the fixation monitoring test parameter is set to blind spot (Heijl-Krakau) mode, proper
fixation is checked by projecting 5% of stimuli at the presumed location of the physiological
blind spot. Only if the patient indicates seeing the blind spot check stimulus will the instrument
record a fixation loss. A high fixation loss score indicates that the patient did not fixate well
during the test, or that the blind spot was incorrectly located.
The printout will show the total number of fixation losses followed by the total number of
stimuli presented within the blind spot. In the example shown in Figure 6.1, the patient had 17
fixation losses out of a total of 26 check stimuli presented.
If fixation losses exceed 20%, “XX” will be printed after the score. When the test is in progress,
the HFA II will beep once if the patient responds to two of the last five fixation checks. If, after
hearing the beep, the patient appears to be fixating properly, you may wish to replot the blind
spot. High fixation loss scores may be due to an erroneously plotted blind spot, caused by
patient head tilt. Straightening the head, or replotting the blind spot, can remedy this situation.
Gaze Tracking may be used as the sole fixation monitor or in conjunction with the Heijl-Krakau
blind spot mode described above. If a patient has demonstrated both good fixation and test
taking reliability in the past, you may prefer using just Gaze Tracking. Because blind spot
monitoring adds time to the test, using the Gaze Tracker alone can shorten test time.
False positive errors Another indication of poor reliability is when a patient responds to catch trials in which no
stimulus has been projected. This is referred to as a false positive response and is tracked as a
false positive error.
The printout will show the total number of false positive errors followed by the total number of
trials. If errors exceed 33% of the trials, “XX” will appear on the screen and on the printout
although test reliability may be compromised at false positive rates that are much lower than
33%. A high false positive score may indicate that the patient is overly concerned about not
seeing all the stimuli. The “trigger happy” person will need to be re-instructed and reassured
that it is normal for many stimuli to be missed. SITA results will not be marked “XX”.
In addition to a high false positive finding, trigger happy patients often show threshold results
that are abnormally high. An example of this phenomenon is shown in Figure 6.1. Any finding
of 40 dB or greater indicates a hypersensitive result which can only be due to patient overreac-
tion or guessing when pressing the patient response button. It is best to note the results early
in the exam and start the test over rather than to allow the test to run to completion and be
completely invalid.
Note: SITA Standard and SITA Fast results will display False Positive errors as percentages,
not fractions, and display this value on the printout only.
EVALUATING
RELIABILITY
6.htp 3/14/03, 3:16 PM 4