Psychometric properties of the Neuropathic Pain Scale (NPS) in a knee osteoarthritis population.

Symptoms resembling neuropathic pain (neuropathic-like symptoms) are prevalent in osteoarthritis (OA) populations. Scales that measure neuropathic-like symptoms frequently were established in groups with true neuropathic pain conditions and have not been assessed in OA. We assessed the psychometric properties of the Neuropathic Pain Scale (NPS) in subjects with OA undergoing total knee replacement (TKR). In a prospective study of adults undergoing TKR for OA, we assessed baseline distributions, acceptability (completion rate), internal consistency (Cronbach's alpha), responsiveness 12 months post-TKR, and construct validity of the NPS. We performed factor analysis and created subscales from the items loading onto each retained factor. We evaluated subscale properties and calculated the proportion of total scores attributable to each subscale and compared this with the proportion expected if each item contributed equally. Mean baseline NPS score among 263 participants was 42.7 (SD: 15.9). Cronbach's alpha was 0.88. Factor analysis produced two factors: "bothersome" (items: intense/sharp/dull/unpleasant/deep; Cronbach's alpha = 0.87), and "dysesthetic" (items: cold/sensitive/itchy/surface; alpha = 0.77). Bothersome items contributed more to total NPS scores (74%) than would be expected if each item contributed equally (50%). NPS scores correlated moderately with baseline pain and function, and decreased after TKR, with standardized response means (SRMs) of: total NPS: 1.77, Bothersome subscale: 2.03, Dysesthetic subscale: 0.70. The NPS had acceptable completion, internal consistency, and construct validity, but was not optimal for use in OA; Bothersome subscale items disproportionately drive total NPS scores and may fail to discriminate between nociceptive and neuropathic-like symptoms.

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The authors declare that there are no conflicts of interest.Appendix Fig. 1a–b. Factor 1/Bothersome subscale scores (0–100 scale) at baseline and 12 months after TKR.Appendix Fig. 1Appendix Fig. 2a–b. Factor 2/Dysesthetic subscale scores (0–100 scale) at baseline and 12 months after TKR.2Appendix Fig. 2Appendix Table 1Subscale item-to-item and corrected item-to-total correlations (Pearson)Appendix Table 1Factor 1 subscaleIntenseSharpDullUnpleasantDeepTotal subscale scoreAlpha with item deleted*Intense1.000.680.490.810.740.830.82Sharp0.681.000.320.610.660.660.86Dull0.490.321.000.470.460.490.90Unpleasant0.810.610.471.000.740.790.82Deep0.740.660.460.741.000.790.82Factor 2 subscaleColdSensitiveItchySurfaceTotal subscale scoreAlpha with item deleted†Cold1.000.460.480.330.500.74Sensitive0.461.000.630.460.650.67Itchy0.480.631.000.460.670.67Surface0.330.460.461.000.500.77*Subscale Cronbach's alpha with all items = 0.87.†Subscale Cronbach's alpha with all items = 0.77.Appendix Table 2Correlations of NPS and NPS subscales with baseline pain, function, and psychosocial measures (Pearson unless otherwise noted; Estimate (95% CI)Appendix Table 2NPS scoreFactor 1 subscale†Factor 2 subscale‡MHIWOMAC painWOMAC functionPCS scoreNPS score–Factor 1 subscale†0.89 (0.87–0.92)–Factor 2 subscale‡0.83 (0.79–0.87)0.52 (0.42–0.61)–MHI−0.31 (−0.42–−0.19)−0.28 (−0.39–0.16)−0.26 (−0.37–−0.14)–WOMAC pain0.66 (0.58–0.72)0.60 (0.51–0.67)0.51 (0.41–0.60)−0.28 (−0.40–−0.16)–WOMAC function0.72 (0.65–0.77)0.68 (0.61–0.74)0.54 (0.45–0.63)−0.31 (−0.42–−0.19)0.77 (0.71–0.82)–PCS score0.40 (0.28–0.51)0.42 (0.30–0.53)0.27 (0.14–0.39)−0.53 (−0.63–−0.43)0.40 (0.28–0.51)0.43 (0.31–0.54)–Pain regions*0.29 (0.17–0.40)0.22 (0.10–0.34)0.25 (0.13–0.36)−0.27 (−0.38–−0.15)0.18 (0.06–0.30)0.24 (0.12–0.36)0.24 (0.10–0.36)*Correlations with pain regions are Spearman correlation coefficients.†The Factor 1 subscale consists of the items intense, sharp, dull, unpleasant, and deep.‡The Factor 2 subscale consists of the items cold, sensitive, itchy, and surface.Appendix Table 3Factor loadings for the 8 NPS-8 items, from the rotated factor pattern matrix and factor structure matrix N = 257Appendix Table 3ItemFactor Pattern†Factor Structure‡Factor 1Factor 2Factor 1Factor 2Sharp0.82*−0.040.80*0.35Hot0.50*0.310.65*0.55*Dull0.390.160.47*0.35Cold0.070.57*0.340.60*Sensitive−0.020.81*0.370.80*Itchy0.000.78*0.370.78*Deep0.84*−0.080.80*0.32Surface0.42*0.380.61*0.59**Asterisk indicates that the item loading on the given factor exceeds 0.4.†In the factor pattern (left), the values are standardized regression coefficients of an item on the factor.‡In the factor structure (right), the values are the product-moment correlations between an item and the factor.