Appendix B: Pass/Refer Criteria - Interacoustics OtoRead Operation Manual

Appendix B: Pass/Refer Criteria

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 measurements 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. As
shown in the figure below, 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 significant hearing loss to 1% or
less. Note: By the binomial distribution, two of three frequencies at
>8.4 dB or three of six frequencies at >7 dB should also ensure less
than 1% probability of passing a moderately-severe hearing-impaired
infant.
OAE trials with newborns 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 section, page 10) 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.
Note: The easiest way to obtain a PASS is to a) make sure the
eartip is fully seated on the probe tip and b) make sure the
eartip is as deeply seated in the ear canal as possible.
Occasional claims of extraordinarily low probabilities of missing an
ear with hearing loss appear to be based on poor statistics. As
discussed by Gorga (Mayo Clinic Teleconference, 1998), since the
incidence of significant hearing loss is roughly 2 per 1000, verifying a
99.7% accuracy would require testing hundreds of thousands of
babies with a given system. Thus to demonstrate that only 3 babies
OtoRead Screener Appendix B: High Frequency DPOAE Page 51