An air-bone gap is defined as which of the following?

• A. Air conduction thresholds are equal to bone conduction thresholds
• B. Bone conduction threshold is 15 dB or more poorer than air conduction
• C. Air conduction threshold is 15 dB or more poorer than bone conduction
• D. There is never an air-bone gap

Answer: (C)

The key idea is the difference between how sound is detected through air and through bone. The air-conduction pathway passes through the outer and middle ear, so problems there raise AC thresholds. The bone-conduction pathway bypasses the outer/middle ear and reflects inner ear function, so BC thresholds stay relatively stable unless there’s a sensorineural issue. An air-bone gap is defined when air-conduction thresholds are 15 dB or more poorer than bone-conduction thresholds. This gap indicates a conductive component to the hearing loss, because the outer or middle ear is impairing air conduction while inner-ear function (as shown by bone conduction) is comparatively better. If AC and BC are the same, there’s no gap and no conductive component. If BC were poorer than AC by 15 dB or more, that isn’t the standard definition of an air-bone gap and would suggest an atypical pattern.