Operating Instructions
63
Appendix D: Pass/Refer Criteria
Pass/Refer Criteria for DPOAE
The decision that a DPOAE exists is based on detecting a signal whose level is significantly above the
background noise level. This requires a statistical decision, since the random noise level in the DPOAE
filter channel can be expected to exceed the average of the random noise levels in the four adjacent
filter channels
—
used as the reference for comparison
—
roughly half the time.
Extended measur
ements of the noise distributions in both the DPOAE filter channel “DP level” and the
rms average of the 4 adjacent channels “N level” indicate that the signal
-to-noise ratio (the difference
between DP and N) has a standard deviation of 5.5 dB. This implies a 10% probability of seeing a 7 dB
SNR simply from the variability of the noise levels in the 2 filter sets.
Requiring an SNR of 6 dB in three out of four frequencies drops the probability of passing an ear with
moderate-to-severe hearing loss to less than 1%. Note: three of six frequencies at >7 dB SNR will also
ensure less than 1% probability of passing a moderate-to-severe hearing-impaired ear.
Pass/Refer Criteria for TEOAE
The same basic principles that underlie DPOAE Pass/Fail criteria underlie TEOAE Pass/Fail criteria. In the
case of transients, requiring SNR of 4 dB at any three out of the six test frequencies drops the
probability of passing an ear with a moderate-to-severe hearing loss to less than 1%.
The SNR limits for transients are lower than the corresponding limits for distortion products primarily
because the traditional noise calculation used in TEOAE measurements (and in the ERO•SCAN
Pro
instrument) gives a 3 dB lower SNR than the calculation used for DPOAEs. Without that difference, the
numerical SNR value for a PASS with the two methods would be quite similar.
Preliminary ERO•SCAN
Pro trials with infants indicate that
the tester’s technique is the single most
important variable in the pass rate on normal-hearing infants. Some testers pick up the technique (see
Operating Instructions
Appendix C
) with only a couple of days’ practice, producing pass rates
comparable to those for other DPOAE equipment they have used for months; other testers take longer.
The ERO•SCAN
Pro uses a novel noise-rejection algorithm (patent pending) that permits accurate
DPOAE and TEOAE measurements in background noise as high as 55-65 dB SPL (A-weighted). Briefly
explained, use of available memory in the ERO•SCAN
Pro processor permits a post-hoc statistical
analysis that identifies those samples whose retention would improve the overall accuracy. Those
samples are included in the final analysis; the noisier samples are rejected.
Note: All manufacturer authorized protocols which provide a Pass/Refer outcome
have been verified in a metal cavity (the equivalent of an ear with moderate-to-severe
hearing loss) to have a less than 1% probability of passing the DPOAE or TEOAE test.
Pass/Refer criteria based on probability statistics of a cavity are independent of age
related normative data (Christensen & Killion, 1999).
Users with the goal of detecting mild hearing loss should ensure that the minimum
amplitude setting is turned on (see section 8.6) and should collect normative data
from the target patient population with the ERO-SCAN Pro to verify the protocol being
used meets the screening goal.
Users creating custom protocols should collect normative data to validate any custom
Pass/Refer criteria.
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