Linking the Spinal Cord Injury-Functional Index (SCI-FI) to the PROMIS Physical Functioning Item Bank

Objective: To link 3 Spinal Cord Injury-Functional Index (SCI-FI) item banks (Basic Mobility, Fine Motor Function, Self-Care) to the Patient-Reported Outcome Measurement Information System (PROMIS) Physical Function (PF) metric.

Design: Observational study SETTING: Six SCI Model Systems rehabilitation hospitals in the United States.

Participants: Adults with SCI (n=855) and healthy individuals (n=730) (N=1585).

Interventions: Not applicable.

Main outcome measures: Three SCI-FI item banks (Basic Mobility, Fine Motor Function, Self-Care), PROMIS PF v1.0 item bank.

Results: SCI-FI item banks (including 30 items from the PROMIS PF item bank) were administered to 855 adults with SCI as part of the original SCI-FI development study. The data were used to attempt to link 3 SCI-FI banks to the PROMIS PF metric via 2 item-response theory methods: fixed-parameter calibration and separate calibration. Sixteen items common to SCI-FI and PROMIS and verified as free of differential item functioning were used as anchor items to implement the methods. Of the 3 banks, only SCI-FI Basic Mobility could be linked with sufficient precision to PROMIS PF. Comparisons of actual vs linked PROMIS PF scores and test characteristic curves suggested the fixed-parameter method provided slightly more precision than the separate calibration method.

Conclusions: The linkage between PROMIS PF and SCI-FI Basic Mobility was considered satisfactory for group-level usage. Score equivalents computed from SCI-FI Basic Mobility will be useful for researchers comparing functional levels in SCI to those observed in other clinical and nonclinical groups (eg, in comparative effectiveness research).