Zeiss Humphrey II-i series, User Manual

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Test Reliability
Humphrey Field Analyzer II-
i
series User Manual 2660021145640 A
6-4
Evaluating Reliability
Even with the most careful perimetric technique, sometimes test results are unreliable. To assist with
evaluating reliability, the HFA II-
i
offers several tools that measure accuracy and consistency. “Catch
trials” are special stimuli (or lack of) which are used for monitoring. When the patient exceeds a set
limit, a warning will display on the HFA screen and print on all types of printouts.
Fixation Losses
When the fixation monitoring test parameter is set to the 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 located incorrectly.
The printout will show the total number of fixation losses followed by the total number of stimuli
presented within the blind spot. If fixation losses equal or exceed 20%, “XX” will be printed after
the score. In the example shown in
Figure 6.1 , the patient had 7 fixation losses out of a total of 14
check stimuli presented.
When the test is in progress, the HFA II-
i
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.
You may use Gaze Tracking 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 or responds faster than is humanly possible. This is referred to as a false positive
response and is tracked as a false positive error. 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.
For SITA tests, the false positive score will be indicated on the printout as a percentage. If false
positive errors equal or exceed 15%, the characters “XX” will appear next to the score on the
printout to call attention to this result. In addition, the message, “Excessive High False Positives” is
printed. A test with 15% or higher false positives will not be able to be used with GPA (see
“GPA
Verifies Exam Reliability,” on page 8-3 ).