Interchangeability of Diverse Analog Scales Used Within the Constant–Murley Score

PURPOSE: To assess the interchangeability of various existing answering scales within the subjective part of the Constant–Murley Score (CMS) and to determine the effect of the different answering scales on the inter- and intraobserver reliability. METHODS: In this prospective, single-center, cross-sectional trial, patients with shoulder problems were included from June to September 2018. Subjects recruited were 18 years or older, presented various shoulder complaints, e.g., diagnosis of osteoarthritis, subacromial pain syndrome, rotator cuff or biceps tendon problems, or frozen shoulder. An extended version of the CMS was prepared including the same questions multiple times but with varying answer scales. Six versions were made with random order of the questions. The answering scales were a verbal and paper based visual analog scale (VAS), smiley face scale, Numeric Rating Scale (NRS), and categories. Internal consistency of the various CMS, Spearman correlation coefficients, intraobserver, and interobserver agreement was assessed (ICC). RESULTS: In total, 93 patients were included. The total CMS using the paper-based VAS, smiley face score, and NRS were 46.9 ± 19.4, 45.2 ± 18.5, and 45.0 ± 18.7. Correlations of the total scores of the different versions varied from 0.98 to 0.99. CMS-category versus CMS-smiley face score and CMS-category versus CMS-NRS pain were significantly different (P = .02 and P = .01). Good internal consistency (0.76-0.79) and acceptable inter- and intraobserver reliability were found (ICC: 0.89-0.97, 0.98-0.99; P < .001). CONCLUSIONS: The different answering scales for the subjective subscales within the CMS for pain, work, and recreational activity were not interchangeable on item level and significantly influenced the total CMS score. Differences were below the smallest detectable change and interpreted as not clinically relevant. Particularly on item level, data from different studies cannot be pooled and compared when different answering scales are being used. The inter- and intraobserver reliability were excellent. LEVEL OF EVIDENCE: Level I, prospective cross-sectional study.