We studied perimetric test results of patients with unilateral
glaucomatous visual field defects to investigate whether FN responses
reflect patient test performance or if they are more dependent on
visual field status. Our results showed that intrapatient differences
in FN frequencies were caused by increases in such responses in
glaucomatous eyes and that they were associated with the amount of
field loss. Patients were included without applying restrictions to any
of the reliability indices.
Originally, the false-negative response parameter was intended to
reflect patient reliability in computerized perimetry.
2 3 Previous studies, however, have observed elevated frequencies of FN
responses in patients with glaucoma compared with normal
subjects.
4 5 6 7 8 If the false-negative rate were primarily an
index of patient performance, similar frequencies would be expected in
both eyes of the same patient, at least in tests performed the same
day. Still, some textbooks consider FN answers to be reflective of a
patient’s inattentiveness,
18 19 20 although some texts
question whether FN answers in patients with glaucomatous visual field
loss are due solely to patient inattentiveness. Lachenmayr and
Vivell,
18 and Anderson and Patella,
3 discuss
the effect of increased threshold variability on FN responses in
glaucomatous visual fields. Weitzman and Caprioli,
26 and
Budenz,
20 discuss the possibility that increased FN
answers may be caused by perimetric visual fatigue. Choplin and
Russell
19 refer to fixation loss as a likely explanation
for increased frequencies of FN in glaucoma eyes. We have previously
demonstrated an association between large test–retest threshold
fluctuations and increased frequencies of FN responses,
27 and, it is likely that the larger frequencies of FN answers in
glaucomatous eyes can be explained by this increased variability of
threshold values. Thus, in glaucomatous eyes, FN catch trials have
considerably less than 100% probability of being seen. The Sita
programs use an improved catch trial method designed to reduce the
number of FN catch trials presented at questionable intensities. The
frequency of FN responses was also smaller with Sita than with the
Humphrey full-threshold strategy, but even with the precautions
included in the Sita program, its false-negative response rate does not
provide a fair estimate of patient inattention.
Thus, our results suggest that current FN catch trial methods are
inadequate for estimating patient attentiveness in perimetry in eyes
with glaucomatous visual field loss. Instead the frequency of
false-negative responses in eyes with glaucomatous visual field defects
is associated with amount of field loss. This means that improved
patient instructions and more intense supervision may not improve test
results.
It would be desirable to have a method for estimation of FN that does
not rely on catch trials, similar to the one used in Sita for
estimation of frequencies of false-positive responses.
28 Unfortunately no such methods are available or have been suggested. The
FN index obtained with catch trials can be considered an indicator of
reliability of test result, with high FN frequencies associated with
low precision in threshold estimates probably explained by the
increased threshold variability in glaucomatous eyes. The field status,
however, provides similar information.