Humphrey FDT User Manual
2660021149532 Rev. B 2015-11
Operation
3-15
Understanding the Threshold C-20 and N-30 Test Results
A plot of the 17 or 19 visual field locations tested will be printed (see samples on the
following pages) and a combination plot will be displayed on the Operator LCD Display for
the THRESHOLD C-20 and N-30 TEST for each eye tested. The first printed results plot will
contain a numerical contrast threshold level in units of dB for each location tested. The
second printed results plot is a deviation plot and will be either clear white or will have one
of four possible levels of shading corresponding age normative significance levels for each
location tested. The results combination plot displayed on the Operator LCD Display will
indicate both the numerical contrast threshold level and the shading for each location
tested.
When reviewing the results of the visual field test, careful consideration must be given to
the reliability indicators (catch trials). The following three indicators appear on the printed
report and on the LCD Display for the threshold test. They are an important measure of
patient reliability in taking the test (and of the reliability of the results). They are indicated
as a ratio of the number responded to the number presented. For example, 1/3 indicates
that the patient responded to 1 of the 3 catch trials presented.
FIXATION ERRORS - The ratio of the number of times the patient responded to a target
placed in the blind spot versus the total number of times fixation was tested (i.e., total
number of targets placed in the blind spot). Six FIXATION catch trials will be randomly
presented for each eye in the C-20 TEST and N-30 TEST. Fixation errors indicate the patient
is not maintaining good fixation during the test, is misaligned, or does not understand the
test.
A
The patient achieved a threshold level in the range that 95% (P > = 5%) of
normal subjects of the same age achieved for the test locations with this
shading.
B
The probability is less than 5% (P < 5%) that a normal subject of the same age
would perform at the threshold level that this patient achieved for the test
locations with this shading.
C
The probability is less than 2% (P < 2%) that a normal subject of the same age
would perform at the threshold level that this patient achieved for the test
locations with this shading.
D
The probability is less than 1% (P < 1%) that a normal subject of the same age
would perform at the threshold level that this patient achieved for the test
locations with this shading.
E
The probability is less than 0.5% (P < 0.5%) that a normal subject of the same
age would perform at the threshold level that this patient achieved for the test
locations with this shading. This shading will also occur if the patient failed to
respond at the maximum contrast level of the instrument (0 dB will be
indicated).
A
B
C
D
E
FIXATION ERRS: 0/0
FALSE POS ERRS: 0/0
FALSE NEG ERRS: 0/0
