Tuesday, March 29, 2016

Your Audiology Tutorial: SRT & WRS

An audiometric battery typically includes a pure tone hearing assessment and speech evaluation, often in two parts: speech reception threshold (SRT) and speech discrimination/word discrimination score (WRS) tests.  Have you wondered what the differences are?

SRT:  A list of bi-syllablic words ("baseball", "outside", etc.) are presented at a comfortable level.  Then the tester drops the level 10 dB until a word is missed.  He or she then raises the level 5 dB until two out of three words are correctly repeated.  This level should be within 5-10 dB of the pure tone average (PTA), or the average of three important speech frequencies where pure tone thresholds (softest level at which one hears a tone 50% of the time) were determined.  Patients often use auditory memory when levels get very soft (relatively speaking).  There are SRT lists that are age appropriate for children ("ice cream", "cowboy", etc.).  If the SRT is not in agreement with the PTA, it may be a red flag to the clinician that either the pure tone results are questionable, or possibly that the patient is malingering (faking a hearing loss to some degree).

WRS: A list of twenty-five monosyllabic, phonetically balanced words ("ball", "kite", "oak") are presented at the most comfortable level (MCL) which is often 30-40 dB above the SRT.  However, patients with a more severe hearing loss may be unable to tolerate such a large increase in intensity and may have an MCL that is only 5-10 dB above the SRT.  The entire list is presented at the MCL.

This discrimination test is scored by percentage; each word is worth 4%, which is deducted for each word missed.  For example, if a patient missed four words, his or her score would be 84%.  When a patient repeats the first ten words correctly, the tester will stop.  Those with hearing sensitivity within normal limits or conductive (middle ear) losses usually do well with this test.  Those with severe inner ear damage often do poorly.  When the WRS score is drastically different between ears it may be indicative of a benign tumor known as an acoustic neuroma.

Rollover may occur when presentation levels are increased, i.e., the patient's percentage score will actually decrease at a louder level.  Louder is not always clearer.  Those with retrocochlear (beyond the inner ear) damage are likely to exhibit this problem.

The WRS is a good preliminary look at success with amplification.  If a patient does poorly at varying levels of presentation, there may be sufficient inner ear damage to make hearing aids a questionable choice.  Often, a clinician will test both ears at the same time to see if the score improves.

Due to hearing loss, some patients cannot repeat any words correctly for either the SRT or WRS tests.  A speech detection test may be implemented, during which the clinician tries to assess the lowest level of a patient's speech awareness, or ability to at least recognize that a word was spoken.  Speaking of...

The above tests can be administered either via live voice or by recorded materials. There are solid arguments for the latter as a patient may see several different audiologists of both genders over the years.  Each tester  may have a different timbre or accent.  Thus, live voice testing may yield some varying results.
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