BS ISO 13091-2:2021 pdf download – Mechanical vibration — Vibrotactile perception thresholds for the assessment of nerve dysfunction Part 2: Analysis and interpretation of measurements at the fingertips
4.3 Test/retest variability of threshold measurements
If the VPT is repeatedly determined at the same fingertip of a subject on separate occasions (e.g. on different days), then the intra-individual test/retest variability in threshold shall be calculated for this subject. The test/retest variability, V(f j ), at a given stimulation frequency or equivalent frequency, f j , (calculated using Formula ) shall be expressed in decibels as one standard deviation from the mean value of the VPTs, expressed in decibels, as determined by repeated measurements. If the VPTs, T(f j ) i , found by repeated measurements at a given stimulation frequency or equivalent frequency, f j , are expressed in dB (ref. 10 −6 m/s 2 ), then:
where T(f j ) M is the mean of n repeated measurements expressed in dB (ref. 10 −6 m/s 2 ).
Under circumstances in which it is not possible to calculate a meaningful standard deviation for a subject (e.g. when only a single measurement is made of a subject’s VPT), then the test/retest variability of the observed VPT shall be estimated for the measurement method used. The estimate shall be derived from the standard deviation of VPTs determined at the fingertips of healthy persons using the same measurement method. The standard deviation shall be based on at least 10 measurements of VPTs performed on separate occasions (e.g. 10 different days). The measurements shall be performed in accordance with the provisions of ISO 13091-1, and the standard deviation, expressed in decibels, shall be calculated from the observed VPTs expressed in decibels using Formula (2). The within subject standard deviation recorded from three or more healthy persons at a given frequency, or equivalent frequency, shall be used as the estimate for the intra-individual test/retest variability at that frequency, or equivalent frequency (one-way analysis of variance).
Normal hormonal changes during the menstrual cycle induce changes of up to 20 dB in the FAII receptor thresholds of females. When estimating the test/retest variability for the FAII thresholds of females, that is, for VPTs at measurement frequencies of 100 Hz, 125 Hz and 160 Hz, this tendency for the threshold to cycle should be taken into account. The threshold changes occur several days before and after ovulation.
4.4 Treatment of unresolved errors Under some circumstances, the examiner might believe that unresolved errors have occurred during threshold measurements. Also, errors might have been introduced by conducting measurements on a defective skin site, as described in ISO 13091-1. In these situations, analysis and interpretation of VPTs using the methods and procedures contained in this document are only possible if additional information is obtained. A second set of measurements shall be performed according to the provisions of ISO 13091-1 if it is believed that more reliable VPTs may be obtained. The second set of VPTs shall be treated as described in this document.
NOTE If, at a single measurement site, the VPTs are determined at two or more frequencies, or equivalent frequencies, mediated by the same mechanoreceptor population, then the consistency of the threshold shifts calculated according to the provisions of 5.6 can be examined to confirm the presence of errors.
4.5 Treatment of suspected increase in test/retest variability Under some circumstances, the examiner might believe that the test/retest variability applicable to the measurement method is not applicable to a subject. The opinion might be based on the lack of consistency in determining ascending and descending thresholds as described in ISO 13091-1:2001, 6.3, or on other information. In these situations, analysis and interpretation of VPTs using the methods and procedures contained in this document are only possible if the variability applicable to the subject is established. A subject specific test/retest variability is established by conducting repeated threshold measurements on the subject according to the provisions of 4.3.
5 Calculation of threshold shift
5.1 General The interpretation of VPTs is facilitated by calculation of the change in observed threshold from a predefined value. The calculation of threshold shift shall be performed for each frequency, or equivalent frequency, and fingertip at which VPTs have been obtained according to provisions of Clause 4.