The Test-retest Reliability of Pain Outcome Measures in People with Phantom Limb Pain

Objectives: To quantify the test-retest reliability of three patient-reported outcome measures of pain for people living with phantom limb pain (PLP) and assess impact of test-retest errors on future research and clinical decisions.

Methods: Thirty-nine participants (30 males), mean (SD) age: 55 (16), mean (SD) years post amputation: 6.8 (8.3), reported their PLP levels on a Visual Analogue Scale (VAS) for pain intensity, the revised Short Form McGill Pain Questionnaire (SF-MPQ-2), and a pain diary, on two occasions 7-14 days apart. Mean systematic change, within-subjects SD, limits of agreement (LOA), coefficient of variation and the intraclass correlation coefficient (ICC) were quantified alongside their respective 95% confidence intervals (95%CI).

Results: Systematic learning effects (mean changes) were not clinically relevant across the VAS, SF-MPQ-2 and pain diary. Within-subjects SDs (95%CI) were 11.8 (9.6-15.3), 0.9 (0.7-1.2), and 8.6 (6.9-11.5), respectively. LOA (95%CI) were 32.6 (26.5-42.4), 2.5 (2-3.3), and 23.9 (19.2-31.8), respectively. ICCs (95%CI) were 0.8 (0.6-0.9), 0.8 (0.7-0.9), and 0.9 (0.8-0.9), respectively, but may have been inflated by sample heterogeneity. The test-retest errors allowed detection of clinically relevant effect sizes with feasible sample sizes in future studies, but individual errors were large.

Discussion: For people with PLP, a pain intensity VAS, the SF-MPQ-2, and a pain diary show an acceptable level of inter-session reliability for use in future clinical trials with feasible sample sizes. Nevertheless, the random error observed for all three of the pain outcome measures suggests they should be interpreted with caution in case studies and when monitoring individuals' clinical status and progress.