The Sensibility Index (SI) is a quantitative measure of the vibration sensibility, measured by a VibroSense Meter. In the skin, there are different types of mechanoreceptors that respond to vibrations of different frequencies. SI is calculated by testing the vibration sensibility at seven different frequencies between 8-500 Hz, which taken together yield a curve - a vibrogram. Changes in the shape of the curve give an early indication of impaired sensibility.
The Sensibility Index (SI) is calculated as the ratio between the integrated areas under the patient curve and the age-matched normal curve. SI below 0.8 is considered pathological and indicative of nerve damage.
The vibrogram curve is automatically compared with a comprehensive age-matched normal population data. Therefore, it is possible to assess how the patients curve relates to the normal curve within the age span. Thus, the SI-value is inherently age-independent.
The SI should primarily be regarded as an average of the vibration sensibility that gives the physician a fast indication of the sensory status. Besides looking at the SI value, the shape of the vibrogram curve should also be analyzed since early sensory disturbances normally appear when the SI still is above 0.8. See also Vibrogram Interpretation.