Optimizing reliability and sensitivity of Semmes-Weinstein monofilaments for establishing point tactile thresholds

Semmes-Weinstein monofilaments (SWM) are widely used to assess tactile point pressure sensitivity. However, the reliability of SWMs has been questioned, standardization of stimulus presentation procedures is lacking, and the sensitivity measure is commonly confounded by the response criterion. This study sought to assess the reliability of two versions of a forced-choice single staircase SWM test with the goal of optimizing test reliability with a minimum number of test trials. Test-retest and intra-test reliability coefficients for SWM threshold values from the plantar halluces of 24 normal subjects were obtained using two versions of a forced-choice single-staircase procedure. One version followed a two-down one-up rule (2D) and the other a three-down one-up rule (3D). The 3D procedure was significantly more reliable than the 2D procedure for all sequential combinations of reversal pairs. A total of four 3D reversal pairs (i.e., eight reversals) were sufficient to achieve test-retest and intra-test reliability coefficients>0.90. High reliability with the minimum number of trials was obtained by calculating the threshold as the mean of eight reversals (test-retest r=0.93, p<0.001; Sessions 1 and 2 intra-test rs=0.87 and 0.92; ps<0.001). Identical median detection thresholds were noted for the two repeated test sessions (5.1g/mm(2)). The threshold values correlated with subject age despite the small range of ages tested, suggesting high sensitivity (Sessions 1 and 2 rs=0.61 and 0.63, ps<0.001). This study demonstrates that SWMs provide highly reliable and sensitive point pressure thresholds with very few trials when an appropriate psychophysical paradigm is employed. The brief forced-choice procedure described herein could serve as a basis for standardizing SWM stimulus presentation methods.