Cognitive Processing Speed Impairment Does Not Influence the Construct Validity of Six-Spot Step Test Performance in People With Multiple Sclerosis
Abstract Objective There is evidence supporting the construct validity of Six-Spot Step Test (SSST) performance as a functional mobility measure in people with multiple sclerosis (MS); however, it is unknown if cognitive processing speed (CPS) impairment confounds the construct validity of SSST performance in MS, as this test seemingly requires intact cognitive processing. The objective of this study was to examine the influence of CPS impairment on the construct validity and precision of SSST performance as a functional mobility measure in MS. Methods Participants (N = 213) completed the SSST, timed 25-foot walk, Six-Minute Walk Test, Timed “Up and Go” Test, Multiple Sclerosis Walking Scale-12, device-measured steps/day, and Late-Life Function and Disability Inventory, Patient-Determined Disease Steps, and the Symbol-Digit Modalities Test (SDMT) as a CPS measure. The overall sample was divided into subsamples with (n = 75) and without (n = 138) CPS impairment based on normative SDMT data. Correlations between SSST performance with other mobility outcomes in the overall sample and CPS impairment subsamples were examined and compared. SSST performance was compared relative to other mobility measures for differentiating MS walking function levels in people with and without CPS impairment. Results SSST performance was significantly and strongly correlated with most mobility outcomes in the overall sample and CPS impairment subsamples. The magnitudes of correlations between SSST performance with most mobility outcomes were not statistically different among subsamples. CPS impairment did not diminish the relative precision of SSST performance for differentiating walking function levels. Conclusions The presence of CPS impairment is not a source of invalidity or imprecision when interpreting SSST performance as a functional mobility measure among people with MS. Impact The SSST is a high-quality endpoint for inclusion in interventions targeting mobility in MS, regardless of CPS status. This information is critical for rehabilitation research and clinical practice given that mobility and cognitive impairment are highly prevalent, co-occurring, and disabling in MS.