Deriving a Barthel Index from the Northwick Park Dependency Scale and the Functional Independence Measure: are they equivalent?

Objective: to examine the extent of agreement between Barthel Index scores derived from Northwick Park Dependency Scores (NPDS) and the Functional Independence Measure (FIM) ratings, in an inpatient setting.
Design and Setting: previously described conversion criteria were applied in a secondary analysis of a large existing dataset, gathered in a tertiary specialist inpatient neurorehabilitation unit.
Subjects: patients with neurological disabilities (N = 1347), mainly following acquired brain injury.
Main Measures: comparison of Barthel scores derived from the NPDS (rated by nursing staff) and from parallel FIM scores (rated by the therapy team).
Results: very strong intraclass correlations were observed between the total scores (0.93, P<0.001); 95% limits of agreement ranged from -3.53 to 4.90. Item-by-item agreement (linear-weighted Cohen's kappa coefficients) ranged from 0.41 to 0.77, which represents 'moderate' to 'substantial' agreement. A significant bias towards lower NPDS-derived scores (median 10 (interquartile range (IQR) 6-16) compared with median 11 (IQR 7-16) for the FIM-derived score; Wilcoxon z 11.60, P<0.001) was considered most likely to reflect actual differences in patient performance observed by therapy and nursing staff.
Conclusions: this study demonstrates good overall agreement between the Barthel Index ratings derived from NPDS and FIM scores. However, scores may be affected by differential performance with nursing and therapy staff, and should not automatically be regarded as equivalent.