Audiogram Interpretation, Baseline Revision, and Standard Threshold Shifts
Standard threshold shift
An STS is an average change of 10 dB or more at 2000, 3000, and 4000 Hz in either ear compared with the baseline audiogram. Two errors dominate: using a single frequency, and comparing against the wrong reference (use the most recent baseline, which may have been revised).
When an STS appears, retest within 30 days under the same conditions. If the shift persists, the employee is referred for evaluation, hearing protection is refit and its adequacy reviewed, and the employer notifies the employee in writing within 21 days. OSHA recordability is a separate test: record the case when the STS average at 2000, 3000, and 4000 Hz in either ear is 25 dB HL or less.
Baseline revision: when the shift is persistent or the reviewer judges the change permanent and not a temporary or non-occupational artifact, the current audiogram may become the new baseline.
Traps
A 10 dB shift at 4000 Hz alone is not an STS. A shift in the worse ear counts. "Retest within 30 days" is not the annual audiogram. An improvement in thresholds is not an STS.
Worked example: baseline 2/3/4 kHz = 5, 10, 15 (average 10). Annual = 20, 20, 25 (average 21.7). Difference = 11.7 dB, so an STS has occurred; the annual average is 21.7 dB HL, which is at or below 25 dB, so it is also recordable.