Test-retest reliability of distortion-product thresholds compared to behavioral auditory thresholds.

When referred to baseline measures, serial monitoring of pure-tone behavioral thresholds and distortion-product otoacoustic emissions (DPOAEs) can be used to detect the progression of cochlear damage. Semi-logarithmic DPOAE input-output (I/O) functions enable the computation of estimated distortion-product thresholds (EDPTs) by means of linear regression, a metric that provides a quantitative estimate of hearing loss due to cochlear-amplifier degradation. DPOAE wave interference and a suboptimal choice of stimulus levels limit the accuracy of EDPTs. This work identifies the test-retest reliability of EDPTs derived from short-pulse DPOAE level maps (EDPT), a method that circumvents limitations associated with both wave interference and suboptimal choice of stimulus levels. The test-retest reliability was compared to that of EDPTs derived from semi-logarithmic I/O functions (EDPT) and that of behavioral thresholds acquired with pure-tone audiometry (PTA) and modified Békésy tracking audiometry (TA) to provide a foundation for identifying and interpreting significant threshold shifts. The DPOAE-based auditory thresholds (EDPT and EDPT) and behavioral thresholds (PTA and TA) were recorded seven times within three months at 14 frequencies with f = 1-14 kHz in 20 ears from ten subjects with normal hearing (4PTA < 20 dB HL). To obtain EDPT, short-pulse DPOAEs were recorded using 21 L,L pairs. Reconstruction of DPOAE growth behavior as a function of L and L using nonlinear curve fitting enabled the derivation of EDPT for each frequency. Test-retest reliability was determined using three different approaches: 1) centered thresholds, 2) average threshold differences, and 3) average absolute threshold differences, between each possible test session (N = 21). Test-retest reliability based on centered thresholds and average threshold differences showed no statistically significant difference between EDPT, EDPT, PTA, and TA for the pooled analysis incorporating all stimulus frequencies. Average absolute threshold differences presented small but significant differences in test-retest reliability with median values of 3.00 dB for PTA, 3.20 dB for TA, 3.34 dB for EDPT, and 3.51 dB for EDPT. A considerable frequency dependence of test-retest reliability was found; namely, the highest test-retest reliability was for EDPT at f = 11 - 14 kHz. Otherwise, at lower frequencies, the highest test-retest reliability was for TA at f =1 - 2 kHz. Overall, the test-retest reliability of EDPT was better than that of EDPT and was similar to that for behavioral thresholds. Hence, deriving EDPT from individual level maps is a promising and sensitive method for objectively monitoring the state of the cochlea. Furthermore, the detection of an equidirectional threshold change at a single frequency in both EDPT and TA might allow reducing the threshold shift as indication of a follow-up examination from the clinical standard of 10 dB down to 5 dB. This stricter indicator might be beneficial when monitoring cochlear damage, for example ototoxicity, in the presence of (remnant) cochlear amplification at baseline.