The 75% ABG Plus Bone Conduction Approach for Hearing Aid Prescription
The 75% air-bone-gap plus bone-conduction (75% ABG + BC) approach is the preferred method for setting hearing aid targets in adults with conductive hearing loss because it appropriately bases the compression ratio on the actual sensorineural component rather than inappropriately using air-conduction thresholds. 1
How the Formula Works
The 75% ABG + BC approach calculates the prescriptive target by:
- Taking 75% of the air-bone gap (the conductive component)
- Adding this to the bone conduction threshold (which represents the true sensorineural hearing level)
- This yields the effective hearing level for programming purposes 1
This method has been the standard approach used by the Australian National Acoustic Laboratories since the release of NAL-NL1 in 1999, replacing the older and less appropriate 25% ABG + AC method. 1
Why This Approach Is Superior
The 75% ABG + BC method is physiologically and audiologically sound because:
It correctly determines compression ratio based on the actual sensorineural hearing loss (represented by bone conduction thresholds), not the artificially elevated air-conduction thresholds that include the conductive component 1
The air-conduction thresholds in conductive hearing loss do not represent true sensorineural damage and therefore cannot serve as the basis for determining appropriate compression settings 1
This approach prescribes gain that restores approximately 75% of the conductive component while accounting for the patient's actual cochlear function 1
Clinical Implications for Maximum Power Output
A critical consideration with the 75% ABG + BC approach is that it requires significantly higher maximum power output (MPO) capabilities from hearing aids, particularly when large conductive components are combined with significant sensorineural loss. 1
Device Selection Considerations:
Receiver-in-aid (RIA) behind-the-ear devices generally have greater output capabilities than receiver-in-canal (RIC) devices and are more appropriate for conductive hearing losses with large air-bone gaps 1
Not all hearing aids will have sufficient MPO to support the output requirements for fitting hearing losses with large conductive components 1
Preoperative assessment of the air-bone gap size helps predict whether conventional amplification will provide adequate benefit or whether alternative options like bone-anchored hearing aids should be considered 2
The Outdated Alternative (25% ABG + AC)
The older 25% ABG + AC approach is not recommended because:
- It inappropriately uses air-conduction thresholds as the basis for compression ratio determination 1
- For conductive losses, AC thresholds do not measure sensorineural hearing loss and lead to incorrect compression settings 1
- Despite being algebraically equivalent in some contexts, these two approaches yield markedly different prescribed gain and required MPO when entered into nonlinear hearing aid prescriptions 1
Practical Application
When fitting hearing aids for conductive hearing loss, clinicians should:
- Use bone conduction thresholds to determine the true sensorineural component and set appropriate compression ratios 1
- Restore approximately 75% of the air-bone gap through amplification 1
- Ensure the selected hearing aid has adequate MPO capabilities, particularly for large conductive components (>40 dB air-bone gap) 1, 2
- Consider that patients with larger air-bone gaps may show poorer results with conventional air-conduction hearing aids and may benefit from bone-conduction alternatives 2
Important Caveats
Individual variation in preferred gain is substantial, even when using the 75% ABG + BC approach—patients with conductive hearing loss show large individual differences in their preferred operating gain 3
Future research may examine whether individuals benefit from or prefer more than 75% restoration of the conductive component, provided adequate MPO capabilities exist to support such restoration 1