It is possible to create a screening protocol in the TEOAE module that provides a test outcome labelled as PASS, REFER or INCOMPLETE.
The application of TEOAEs in screening is very different to that when used for diagnostic purposes. It is therefore extremely important to select the correct test settings or protocol, depending on the patient or expected use of the test results.
As a screening protocol is typically used for the purpose of differentiating patients with likely normal hearing from those with a peripheral dysfunction, it is important to create a protocol that has only a small probability of providing a pass on a test, for patients with a moderate or greater hearing impairment.
In newborn hearing screening, test protocols typically require that 3 out of 4 bands (in the speech frequency range) are detected for a PASS result. When the newborn passes the hearing screening test, it indicates that there is a significantly low chance that they have a significant auditory disorder requiring amplification (e.g., a hearing aid or cochlear implant).
A REFER result only indicates that the detection criteria of the test protocol was not met. This could be due to test conditions (i.e., too noisy), a hearing loss (conductive or sensorineural) or poor testing techniques.
Infants that REFER on a screening test may be rescreened before being referred to a diagnostic audiology department for more thorough testing to rule out a permanent hearing loss.
Ensure that the pass-refer protocol that you create is valid for the test population and for the expected clinical outcome. For more comprehensive information about outer hair cell function and for clinical based treatment and rehabilitation decisions, a diagnostic OAE protocol should be used in conjunction with other diagnostic audiological tests.
A protocol used for screening purposes has the purpose of differentiating patients with likely normal hearing from those with a peripheral dysfunction. Therefore, it is important to create a protocol that has only a small probability of providing a pass on a test, on patients with a moderate or greater hearing impairment.
The following test parameters should be carefully defined to create a highly sensitive pass-refer protocol.
Select or create an appropriate test protocol that includes the cochlear region of interest. For TEOAEs, half octave bands centered around the major audiometric test frequencies are typically used (1000, 1500, 2000, 3000, and 4000 Hz). The number of bands included in the protocol should also be considered. In newborn screening, the 1000 Hz center frequency band is typically omitted due to ambient and physiological noise affecting test outcomes.
A non-linear click stimulus is typically used in both screening and diagnostic OAE testing. For screening it is strongly advised to use the non-linear click stimulus since it is optimized for maximized sensitivity.
Typical TEOAE screening and diagnostic test protocols use stimulus levels between 80 – 84 dB SPL. Stimulus levels lower than this are likely to produce more REFER outcomes on ears with normal hearing sensitivity and stimulus levels higher than 84 dB SPL should be avoided to reduce the chance of giving a PASS ears with a mild-moderate hearing impairment.
Accurate presentation of the stimulus to the ear is important to produce valid OAEs. A stimulus tolerance value (dB) defines the allowed difference of the stimulus presentation level in both a positive and negative direction. Setting an appropriate stimulus tolerance level will ensure that a warning is provided to the tester when unwanted stimulus level changes occur that could affect test outcomes.
Changes in stimulus levels during testing usually occur due to probe movement in the ear canal. Probe movement can be reduced by ensuring a secure probe fit before testing and instructing the patient to remain still during testing.
Applying a high pass filter assists in the collection of data during noisy conditions, helping to remove low frequency noise. Filtering should be used carefully as it not only filters out the noise, but the OAE response.
Example: Setting the high pass filter to 1200 Hz will attenuate not only the noise below this frequency, but also the OAE response. This may produce a reduction in OAE amplitude below 1200Hz (e.g., in the 1000 Hz band).
As ambient and physiologic (patient) noise levels have a huge effect on OAE recordings, assessing the noise levels in the test environment before and during testing is extremely important, especially in nonideal test environments such as in a newborn nursery.
In TEOAE testing, a typical noise rejection setting is between 47 - 55 dB SPL. This ensures that sweeps recorded in conditions where the background noise is high are rejected and not included in the averaging of the response. While it lengthens the test time, it is not recommended to turn the acceptable noise level off. Sweeps recorded with high levels of noise can reduce the integrity of the data being analyzed.
A suitable test time should be selected to ensure that enough time can be spent to extract the TEOAE response from the surrounding noise in the recording. This can usually be set as either an absolute test time (e.g., 1 – 6 minutes) or as a maximum number of sweeps (e.g., 2000 sweeps). Testing normally continues until the maximum test time is reached or the required number of TEOAE bands are detected, whichever happens sooner.
Screening protocols typically use criteria of 2 or 3 out of 4 bands for a PASS result. Setting the required numbered of bands too loosely (e.g., 1 out of 4 for a PASS) would increase the chances of passing an ear with a hearing loss, whereas setting it too stringently (e.g., 4 out of 4 for a PASS) would increase the number of REFER outcomes.
A mandatory band can be set for inclusion in screening protocols. For example, if a 2 out of 4 bands for a pass protocol was used, the 1000 Hz band could be set as a mandatory band for inclusion. This means that if during testing, 2000 and 4000 Hz were detected, testing would continue until 1000 Hz was detected or the test timed out.
This setting defines the minimum number of sweeps that must be recorded before the test meets the detection criteria. It is usually recommended to collect a minimum of 80 sweeps to ensure enough good data can be analyzed to form a conclusion.
OAE amplitudes are generally in the range of -10 dB SPL to +30 dB SPL in healthy functioning ears. Therefore, setting a minimum OAE level ensures that low-level artifact responses are not accepted as a true OAE response, even when the required SNR is met. Minimum OAE levels should not be set lower than device specific system distortion levels.
The total OAE value is the total energy level of the correlated A and B response waveforms. It is obtained from the overall FFT response, not just the half octave frequency bands of interest. In other words, it gives an indication of the amplitude of the total obtained OAE response. Some newborn hearing screening protocols stipulate a minimum total OAE of 0 dB as part of the stop criteria.
The recording window range defines the time range from which the OAE is analyzed following the presentation of each click stimulus. The TEOAE440 uses a minimum start time of 4 ms to ensures that any analysis of the OAE is not performed during presentation of the stimulus or the potential artifact that follows. A short recording window of approximately 8 - 12 ms allows for more rapid presentation of the click stimulus meaning faster acquisition of OAE data.
The benefits of a short recording window are:
Using a shorter recording window results in a reduction of the lower frequency portion of the response and should not be used when trying to record TEOAEs below 1000 Hz. A longer recording window of approximately 20 ms should be used when trying to assess the broadest range of cochlear hair cell function.
Stimulus presentation becomes slower meaning slightly longer test times to collect the same amount of sweeps when compared to using a short recording window. A long window should always be selected when wanting to assess TEOAEs below 1000 Hz.
The signal-to-noise ratio (SNR) refers to the difference in dB, between the level of the OAE response and the background noise. The SNR can be thought of as an estimate of the reliability with which the OAE response level has been estimated. When SNRs are high, the contribution of the noise in the recording is low and there is more certainty that the displayed OAE response is true.
The SNR is calculated from two variables – the OAE response (or signal) generated by the cochlea and the noise, a random variable unrelated to cochlear status. The signal (OAE response) should remain constant during averaging, while the noise level should decrease as test time increases. Most literature recommends a minimum SNR of 6 dB in addition to other criteria to determine the presence of a valid OAE response.
To avoid undertaking unfeasibly large clinical trials involving say 10,000 infants, some countries that administer UNHS programs stipulate test criteria for a sensitivity assessment. The following is an extract from the United Kingdom Newborn Hearing Screening (NHS) program tender document:
"Evidence of cavity trials at volumes close to 0.05; 0.1; and 0.2ml; showing a maximum of 1 pass on 120 repetitions. To be conducted with normal stimulus levels present in (a) quiet conditions and (b) with wide bandnoise applied externally to such a level that the reject system is activated between 30% and 70% of the time. Such noise may be generated by the wide band masking of a clinical audiometer. For condition (b) if a method of data rejection is employed that does not enable this test to be performed, provide evidence of an equivalent test in noisy conditions."
Based on the above information, a user defined screening protocol’s sensitivity could be assessed by the following method:
The final sensitivity estimate is calculated by expressing the number of ‘true refers’ as a percentage of the total number of tests. When tests are performed in a standard test cavity, we would hope that all test outcomes were a ‘refer’ and therefore the test sensitivity would be measured at 100%.
However, statistically it is not safe to use this value unless the number of tests is very large. Instead, the sensitivity is assessed assuming a worst case scenario such that if ‘n’ tests are performed, if there was an ‘n+1’ test it would produce a ‘false pass’.