The fitting process: an overview

A complete hearing aid fitting involves far more than selecting a device and programming it to an audiogram. Best-practice fitting encompasses pre-fitting assessment, prescriptive target selection, electroacoustic verification, real-ear measurement, speech perception testing, counseling, and outcome validation — a process that may span multiple appointments and requires ongoing follow-up.

The American Academy of Audiology's Clinical Practice Guidelines for Adult Hearing Aid Fitting (2023) and the AAA Pediatric Amplification Guidelines provide the evidence-based framework for fitting practice in the US. The International Hearing Aid Fitting Forum (IHAFF) and the Ida Institute also publish widely-used clinical resources.

Prescriptive fitting targets

Prescriptive fitting targets define the desired hearing aid output — the gain and frequency response that should be delivered to the ear canal — based on the patient's audiogram. The two most widely used prescriptive methods are:

• NAL-NL2 (National Acoustic Laboratories Non-Linear 2): Developed by the Australian National Acoustic Laboratories. NAL-NL2 targets maximize speech intelligibility while maintaining overall loudness at a comfortable level. It is the most widely used prescriptive method globally and is the default in most major fitting software platforms.
• DSL v5.0 (Desired Sensation Level): Developed at Western University (Canada). DSL targets audibility across the full speech spectrum, with a particular emphasis on ensuring that soft speech is audible. DSL is widely used in pediatric fitting and for patients with severe to profound loss.

Both methods have strong evidence bases. The choice between them depends on patient characteristics, degree of loss, and clinical judgment. Many audiologists use NAL-NL2 as a starting point for adults and DSL for pediatric patients, then adjust based on patient feedback and real-ear verification.

Real-ear measurement (REM)

Real-ear measurement (REM) — also called probe microphone measurement — is the gold standard for verifying that a hearing aid is delivering the prescribed gain to the patient's ear canal. Despite strong evidence supporting its use, surveys consistently show that fewer than 30% of hearing aid fittings in the US include REM.

REM involves placing a thin probe microphone tube in the ear canal alongside the hearing aid and measuring the sound pressure level at the eardrum while the hearing aid is active. The measured output is compared to the prescriptive target, and the hearing aid is adjusted until the output matches the target across frequencies.

Key REM measures include:

• REAR (Real-Ear Aided Response): The output of the hearing aid in the ear canal with the hearing aid active.
• REIG (Real-Ear Insertion Gain): The difference between the unaided and aided ear canal response — the gain provided by the hearing aid.
• RECD (Real-Ear-to-Coupler Difference): The difference between the ear canal response and a standard 2cc coupler response. Used to convert coupler measurements to predicted ear canal values, particularly important in pediatric fitting.
• RESR (Real-Ear Saturation Response): The maximum output of the hearing aid in the ear canal, used to verify that the OSPL90 does not exceed the patient's loudness discomfort level.

The AAA Clinical Practice Guidelines recommend REM for all hearing aid fittings. The evidence consistently shows that fittings verified with REM produce better audiometric outcomes and higher patient satisfaction than fittings without REM.

Speech perception testing

Speech perception testing in the aided condition provides functional evidence of hearing aid benefit beyond audiometric thresholds. Commonly used measures include:

• Word recognition scores (WRS): Monosyllabic word lists (NU-6, CID W-22) presented at conversational level in quiet. Useful for documenting aided benefit but limited by ceiling effects in patients with good word recognition.
• QuickSIN (Quick Speech-in-Noise): Measures speech understanding in noise using sentence materials at varying signal-to-noise ratios. Provides a signal-to-noise ratio loss (SNR loss) score that predicts real-world communication difficulty and guides counseling about realistic expectations.
• BKB-SIN: Pediatric version of the QuickSIN, using Bamford-Kowal-Bench sentences.
• HINT (Hearing in Noise Test): Measures the SNR required for 50% sentence recognition in noise.

Outcome validation

Outcome validation measures document the real-world benefit of hearing aids from the patient's perspective. Validated self-report measures include:

• HHIE/HHIA (Hearing Handicap Inventory for the Elderly/Adults): 25-item questionnaire measuring the social and emotional impact of hearing loss. Widely used for documenting pre- and post-fitting benefit.
• APHAB (Abbreviated Profile of Hearing Aid Benefit): 24-item questionnaire measuring hearing aid benefit across four subscales: ease of communication, reverberation, background noise, and aversiveness of sounds.
• COSI (Client Oriented Scale of Improvement): Patient-centered measure where the patient identifies up to five specific listening situations they want to improve. Outcomes are rated at follow-up. Highly individualized and useful for counseling.
• GHABP (Glasgow Hearing Aid Benefit Profile): Measures benefit, residual disability, satisfaction, and use across four initial situations and up to four patient-nominated situations.
• IOI-HA (International Outcome Inventory for Hearing Aids): Seven-item questionnaire covering use, benefit, residual activity limitation, satisfaction, residual participation restriction, impact on others, and quality of life.

Pediatric fitting considerations

Pediatric hearing aid fitting requires additional considerations beyond adult protocols:

• RECD measurement is essential for accurate target calculation in children, whose ear canal volumes differ significantly from adults and from the standard 2cc coupler.
• DSL v5.0 targets are preferred for pediatric fitting, particularly for infants and young children where audibility of soft speech is critical for language development.
• Behavioral audiometry (VRA, CPA) provides threshold data for fitting when pure-tone audiometry is not possible.
• ABR-derived thresholds may be used for fitting in infants when behavioral thresholds are not available, with appropriate correction factors.
• Frequent follow-up and re-verification are essential as ear canal volume changes with growth.
• Family counseling and training are as important as the fitting itself — caregivers must understand how to insert, maintain, and troubleshoot the hearing aids.