Welch Allyn OAE Hearing Screener - User Manual - Manual (Page 59)

Appendix D: 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 50% of 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 SNR
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 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.
Preliminary 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 “Prepare patient for testing” ) 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.
Occasional claims of extraordinarily low probabilities of not detecting 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 1,
000, verifying 99.7% accuracy would require testing hundreds of thousands of babies
with a given system. To demonstrate that only 3 babies out of 1,000 with hearing loss
were not detected would require follow-up testing of 500,000 babies. To our knowledge,
no one has performed such tests to date.
Directions for use Appendices 55