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Without hesitation we cordially thank Dr Verbeek1 for his comments on our article about temporary threshold shift (TTS) at 4 kHz as a predictor of noise induced hearing loss (NIHL).2 This gives us the opportunity to discuss some important issues in greater detail.
Dr Verbeek correctly mentioned that we have chosen to predict hearing level rather than hearing loss. This choice was taken due to the fact that the hearing level is what guides the decisions in occupational examinations. However, from a more fundamental perspective, hearing loss would definitely be the more important parameter to predict. Especially if workers with previous noise exposure and some hearing deterioration are concerned, it is of importance to predict future development of hearing loss under continued noise exposure. Indeed, in the regression analysis summarised in table 2 of our article,2 baseline hearing level had a significant relationship to the hearing level at end of follow-up.
In order to address the issue brought up by Dr Verbeek, we analysed the data with hearing loss as the outcome variable. The results are shown in table 1. It can be seen that the TTS at 4 kHz from test exposure is also a highly significant predictor of hearing loss. The correlation is even stronger as compared to that reported for hearing level at end of follow-up. Also noise years remained statistically significant with little change in regression coefficient. However, the coefficients for hearing protector use became smaller and were no longer statistically significant. Inspection of the correlation between hearing protector use, baseline levels and hearing loss revealed that baseline levels already had a negative correlation with hearing protector use. Workers that tended to less frequent use had higher baseline levels (higher pre-exposure hearing loss), indicating that they had more noisy leisure time activities (because at the initial test they were apprentices without previous occupational noise exposure).
Overall the reanalysis of our data demonstrates that TTS from test exposure also predicts hearing loss as well as hearing level.
While we concur with the assessment of Henderson et al,3 mentioned by Dr Verbeek, that in general TTS as predictor of hearing loss cannot be advocated, we maintain that the specific test we proposed, that is, to consider the TTS peak at 4 kHz that occurs quite independently of exposure frequency, is suitable to assess susceptibility to NIHL.
Of course, every noise exposure, including an exposure from a 20 min TTS test, can have consequences on the integrity of the auditory system. But compared to the years of occupation in noisy occupational settings with hours of daily exposure, we do not think it unethical to perform such a TTS test that may prove useful in individual counselling. In our opinion it is more persuasive evidence to the individual worker if his or her reaction to the exposure can be used during instructions about hearing protector use and health risks of noise exposure as compared to general information only.
Competing interests None.
Provenance and peer review Not commissioned; internally peer reviewed.
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