Normal ABR Threshold in Infants
Normal ABR thresholds in infants are approximately 20-30 dB nHL or less, with frequency-specific variations: 500 Hz averages 23-24 dB nHL, while 1000-4000 Hz range from 6-15 dB nHL 1, 2.
Frequency-Specific Normal Values
The American Academy of Pediatrics guidelines establish that air-conduction hearing threshold levels greater than 20 dB at any frequency indicate possible hearing loss and warrant referral to a pediatric audiologist 1. However, diagnostic ABR testing in normal-hearing infants reveals more nuanced frequency-specific patterns:
- 500 Hz: 23.6-23.8 dB nHL (mean values from multiple studies) 3, 2
- 1000 Hz: 14.4 dB nHL 3
- 2000 Hz: 6.0-12.9 dB nHL 3, 2
- 4000 Hz: 7.0-12.6 dB nHL 3, 2
The lower frequencies (particularly 500 Hz) consistently show higher thresholds than mid and high frequencies in normal infants, which is a critical clinical consideration when interpreting results 2, 4.
Age-Related Considerations
Most (92-100%) infants with normal hearing demonstrate ABR responses to 30 dB nHL tones 2. Research shows that ABR thresholds in infants under 6 months average 9.3 dB nHL for click stimuli, which is slightly higher than adult values, reflecting auditory pathway maturation 5. By 6-18 months, infant thresholds approach adult levels.
Important clinical pearl: Approximately 85% of normal newborns have ABR thresholds less than 40 dB nHL, 54% less than 20 dB nHL, and 15% fall between 40-60 dB nHL 6. This higher threshold group likely represents transient conductive factors common in newborns rather than true hearing loss.
Clinical Application Algorithm
When interpreting infant ABR results:
Use 30 dB nHL as the screening cutoff - failure to respond warrants comprehensive diagnostic evaluation 2, 6
Apply frequency-specific interpretation:
- Low frequencies (500 Hz): thresholds up to 25 dB nHL may be normal
- Mid-high frequencies (1000-4000 Hz): thresholds should be ≤15 dB nHL
Consider the 20 dB rule from AAP guidelines as the conservative threshold for referral 1
Account for test methodology: Tone-pip ABR provides lower (more accurate) thresholds than ASSR, with differences of 8-15 dB depending on frequency 4
Critical Caveats
ABR is not a true test of hearing - it assesses auditory pathway structural integrity but does not evaluate cortical processing 1. Even with normal ABR, behavioral audiometry remains the gold standard once the child can cooperate 1.
The correlation between ABR (dB nHL) and behavioral thresholds (dB HL) varies by frequency: at 500 Hz, ABR overestimates by approximately 8-9 dB; at 2000-4000 Hz, ABR may underestimate by 0-4 dB 2. Overall, 93% of ABR thresholds fall within 20 dB of behavioral thresholds 2.
For any infant showing "no response" on tone-burst ABR, click-evoked ABR must be performed to rule out auditory neuropathy spectrum disorder, particularly in infants with risk factors like hyperbilirubinemia 7.