Minimal Important Differences and self-identifying treatment response in chronic inflammatory demyelinating polyneuropathy.

INTRODUCTION The use of outcome measures is recommended for chronic inflammatory demyelinating polyneuropathy (CIDP). Implications of minimal important differences (MID) to ascertain responder status are unknown. The reliability of patient-reported treatment-response in relation to clinically-relevant change is also unknown. METHODS We retrospectively studied 72 subjects with "definite" or "probable" CIDP evaluated at pre-specified time-intervals pre- and post-treatment. We derived MID and the minimum detectable change with 95% confidence intervals (MDC95 ) for four scales. Scale sensitivities were determined with applicable MID-defined cut-offs (aMIDc), to detect subjects with self-identifying treatment response through a single question. RESULTS The use of MID was not valid for the Medical Research Council (MRC) Sum Score, as MDC95 >MID. The aMIDc for the Overall Neuropathy Limitation Score (ONLS) was 1 (sensitivity: 84.7%). The aMIDc for the centile Inflammatory Rasch-built Overall Disability Scale (cI-RODS) was 8 (sensitivity: 62.3%). The aMIDc for grip strength was 4kg (sensitivity: 79.1%). MID-defined amelioration of any one scale amongst ONLS, cI-RODS or grip strength, significantly improved sensitivity to detect treatment-responders compared with the ONLS alone (McNemar's Test: p=0.008, OR: 3.36 [95% CI: 1.44-7.86]). Patient-reported improvement was highly reliable in relation to MID-defined amelioration on any one scale. DISCUSSION In subjects with CIDP, MID-defined amelioration of any one of 3 commonly used outcome measures offers optimum relevance and sensitivity to detect self-identifying treatment-responders. Patient reliability to single-question ascertainment of response is high in relation to MID-defined clinical relevance. These findings support use of multiple outcome measures in CIDP monitoring and justify enhanced patient involvement in the process.