Validity of estimating the 3 KHz audiometric threshold by averaging the thresholds on 2 and 4 KHz in cases of Noise-induced Hearing Loss

Background: Hearing thresholds at 3000 Hz are generally not measured in routine clinical audiometry. However, for purposes other than clinical diagnosis, the threshold at 3 kHz has many applications, in epidemiological studies in the field of occupational health and medicine, as well as in (medicolegal) quantification of physical impairment due to hearing loss, particularly noiseinduced hearing loss (NIHL). The present study addressed the validity of estimating, in the case of NIHL, the 3 kHz-audiometric thresholds by averaging the thresholds at 2 and 4 kHz. Methods: All 200 investigated subjects (400 ears) had a well-documented noise exposure, moderate to severe NIHL, and underwent, as they were claiming for compensation, a detailed medicolegal audiological investigation, including beside pure tone audiometry, electrophysiological objective frequencyspecific threshold definition using cortical evoked response audiometry (CERA) and auditory steadystate response (ASSR). Results: The study results showed a good correlation between the 2-4 kHz interpolation and the actual 3 kHz threshold; the error may be around 2 dB on average. However, in individual cases, the results demonstrated that the error due to interpolation exceeds 5 dB HL in about onequarter of the cases. This error is predictable; the larger the 2-4 KHz difference (which reflects the steepness of the left slope of the audiometric notch), the larger the error (on either side) made by interpolating. Conclusion: For epidemiological studies with large amounts of data, the interpolated threshold (average of 2 and 4 KHz) may be considered as a valid estimate of the true value of the 3 KHz threshold. More caution is required in individual cases: the error due to interpolation exceeds 5 dB HL in about onequarter of the cases, but this error is predictable.