Background We previously described the construction of a measure for global strength in MS. However, loss of strength is common to many diseases. Thus, a generic strength measure may prove widely applicable and allow inter-disease comparison.

Aim To determine whether measurement performance of the MS-derived strength scale remained stable in people with CIDP.

Method MRC strength data from the RMC study (n=238) were amalgamated with those from our previously described MS sample (n=371). Items included were the eight pairs of muscles (four upper limb; four lower limb) consistent across both studies. Rasch analysis was used to determine the extent to which the 16-item scale met expected criteria for measurement.

Results Analysis indicated muscles mapped a measurable range of global strength. However, 9/16 muscles had statistically significant differential item functioning (DIF). In these muscles, mean scores in groups of equally strong people varied between diseases by up to 2.8 MRC grades. Analysing disease-specific data separately suggested muscle locations on the “strength ruler” changed in hierarchy depending on the disease measured.

Conclusion Summed strength scores should not be compared across diseases. Rather, scores for different diseases should be analysed in the context of their own muscle hierarchies using a vehicle such as Rasch analysis. However, this study does identify a group of nondiscriminatory muscles on which a generic strength scale may be constructed.