Plateau of practice effects and noise with repeat SDMT testing and in multiple sclerosis.

Background: The Symbol Digit Modalities Test (SDMT) is the most widely used test of cognition in people with multiple sclerosis (PwMS), and repeated testing is confounded by test-retest noise and practice effects.

Objective: To investigate the extent to which SDMT practice effects build and plateau with high-frequency testing, reliable cutoffs for longitudinal change, and whether short-interval testing improves detection of cognitive decline.

Methods: PwMS were tested with the SDMT monthly across 3 years. Plateau regression analyses were used to determine inflection points of practice effects, and the reliable-change was assessed. To evaluate effects of testing density on cognitive decline detection, this sample was compared with a separate low-density testing group.

Results: The study included 71 people with relapsing-remitting MS (77.5% female), mean (standard deviation (SD)) age 37.3 (9.3), with 27.8 (21.3) SDMT assessments over 3.2 (2.4) years. The plateau of practice effects was reached after 18 repetitions (p < 0.001). Within this sample, ⩾7-point SDMT change was needed to detect cognitive decline with 90% confidence. Higher testing density did not improve detection of cognitive decline (p = 0.256).

Conclusion: We observed practice effects building for eighteen SDMT assessments and test-retest variability consistent with literature. These results provide guidance on SDMT which should be accounted for with alternate versions and reliable-change methodologies.