Calculating Percent Air-Bone Gap from Pure-Tone Audiometry
The percent air-bone gap is not a standard audiometric calculation used in clinical practice. Audiologists and otolaryngologists measure the air-bone gap (ABG) in decibels (dB), not as a percentage, to characterize conductive hearing loss components 1, 2.
Standard Air-Bone Gap Measurement
The air-bone gap is calculated by subtracting the bone conduction threshold from the air conduction threshold at each frequency, expressed in decibels. 1, 2
Calculation Method:
- ABG (dB) = Air Conduction Threshold (dB HL) - Bone Conduction Threshold (dB HL) at each test frequency 1
- Air conduction thresholds should be measured at 250-8000 Hz 1, 3
- Bone conduction thresholds should be measured at 250-4000 Hz 1
- A clinically significant ABG is typically ≥10 dB, though some sources use ≥15 dB or ≥20 dB as thresholds 4, 5
Important Clinical Considerations
Normal Ears Show Minimal Air-Bone Gaps
- In listeners with normal hearing and normal middle ear function, the ABG should average 0 dB across frequencies. 6
- Small ABGs of -1.7 to 0.3 dB at 0.5-2.0 kHz are normal 7
False Air-Bone Gaps at 4 kHz
- A common pitfall is the false ABG at 4 kHz, which averages 10.6 dB in normal-hearing listeners and 14.1 dB in sensorineural hearing loss patients. 7
- This false ABG results from incorrect reference threshold levels in audiometer standards, not true conductive pathology 6, 7
- The 4 kHz ABG increases with hearing loss severity, ranging from 10.1 dB with mild loss to 21.1 dB with severe loss 7
- Clinicians must avoid misinterpreting these false ABGs as conductive pathology requiring medical or surgical intervention. 6
Distinguishing Conductive from Sensorineural Loss
When bone conduction testing is unavailable, conductive hearing loss can be differentiated from sensorineural loss using air conduction audiometry combined with:
- Tympanometry to confirm middle ear function 1, 3
- Speech recognition testing, as conductive losses typically preserve speech discrimination 4, 5
- Low-frequency pure-tone average combined with digits-in-noise testing (97.2% sensitivity, 93.4% specificity) 5
Why Percentage Calculation Is Not Standard
The air-bone gap represents an absolute difference in sound transmission pathways and is most clinically meaningful when expressed in decibels rather than percentages. 1, 2
- Decibel measurements directly reflect the degree of conductive component requiring treatment 1
- Grading systems for hearing loss (Chang, Brock, CTCAE) all use absolute dB thresholds, not percentages 1
- Treatment decisions for conductive pathology depend on the magnitude of ABG in dB, not relative percentages 1
Conductive Loss Must Be Excluded for Ototoxicity Grading
- When grading ototoxicity or sensorineural hearing loss, conductive losses must be excluded by confirming normal tympanograms or using bone conduction thresholds. 1
- If only air conduction is available, normal middle ear function must be confirmed with tympanometry before applying sensorineural grading criteria 1