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Reliability, Coefficient of Variation of standardized sNCT^® CPT^® Measures

The Coefficient of Variation is a statistical measure that expresses the reliability of repeated test measures. Sensory nerve functioning in healthy subjects normally varies moderately, minute by minute and throughout the day, and sensitive tests will accurately reflect those minor variations. The table below presents the findings from five different studies that evaluated repeated measures to determine the Coefficients of Variation for the sNCT CPT measures. The heading for each column of data reflects the country in which the study was conducted and study numbers below refer to the publications cited below.

Healthy Subject % Coefficients of Variation

CPT

Frequency

Japan(1)

USA(2)

UK(3)

Hungary(4)

Japan(5)

5 Hz

15-27

28

16-20

18

27

250 Hz

11-12

14

-

12

23

2000 Hz

5-6

7

8-11

8

14

The above measures were obtained using the double-blind forced choice testing procedure with a resolution of +/- 20 µAmp, p<0.006. 

The Coefficients of Variation of sNCT/CPT measures are consistently highest at the lower frequencies of stimulation. Since healthy sNCT/CPT measures are always lower at the low frequencies than they are at the higher frequencies, minor variations show up as a larger percentage of the lower frequency CPT measures.  This is a linear distortion a logarithmic  phenomena.  Researchers at the Japanese National Institute of Longevity Sciences reported that on average, over 90% of repeated CPT values at any frequency vary less than 20 CPT units for both intra- and interrator reliability (n=13) each group.  There was a significant correlation between all measures (p<0.001, CPTs are obtained on a scale from 1 to 999, 1 CPT unit = 10 microAmperes), reference (1) below.

The references for the above cited data are:

(1) Japan - Takekuma, K., Ando, F., Niino, N., Shimokata, H. Age and gender differences in skin sensory threshold assessed by current perception in community-dwelling Japanese, Journal of Epidemiology, Volume10(1):S33-S38, 2000.

(2) USA - Katims, J.J., Rouvelas, P., Sadler, B.T., Weseley, S.A. Current Perception Threshold: Reproducibility and Comparison with Nerve Conduction in Evaluation of Carpal Tunnel Syndrome. Transactions of the American Society of Artificial Internal Organs, Volume 35(3):280-284, 1989.

(3) UK- Masson, E.A., Veves, A., Fernando, D., Boulton, A.J.M. Current perception thresholds: a new, quick, and reproducible method for the assessment of peripheral neuropathy in diabetes mellitus. Diabetologia, Volume 32:724-728, 1989.

(4) Hungary - Barkai, L., Kempler, P., Vámosi, I., Lukács, K., Marton, A., Keresztes, K. Peripheral Sensory Nerve Dysfunction in Children and Adolescents with Type 1 Diabetes Mellitus. Diabetic Medicine, Volume 15:228-233, 1998.

(5) Japan - Umezawa, S., Kanamori, A., Yajima, Y., Aoki, C. Current Perception Threshold in evaluating diabetic neuropathy. Diabetes (Japan) Volume 8(1):711-719, 1997.