Exploring the Structured Inventory of Malingered Symptomatology in Patients with Multiple Sclerosis

Greater attention is being paid to performance validity in patients with multiple sclerosis (MS), though limited work has examined symptom validity measures in this population. Extensive previous literature has examined the Structured Inventory of Malingered Symptomatology (SIMS) across diverse populations with variable results. The purpose of the present study was to determine the extent to which the SIMS and/or its associated variables/items are useful in detecting non-credible cognitive performance in patients with MS. Sixty-seven patients with MS (Mage = 48.64; 71.6% women; 83.6% White) were referred for neuropsychological evaluation within a large, Midwestern academic medical center. Participants were categorized into credible (n = 50) and non-credible (n = 17) groups based on standalone performance validity test (PVT) criteria. The non-credible group reported significantly higher SIMS total scores than the credible group (d = 1.69). Results revealed that SIMS total score was broadly elevated across the entire sample (M = 17.55, SD = 8.30) and significantly correlated with depressive (r = .56) and anxiety (r = .38) symptoms. Publisher-recommended cutoffs for SIMS total scores of > 14 (specificity = .50) and > 16 (specificity = .64) yielded unacceptable levels of false positives. A novel subscale (SIMS-MS) was created from six items that were endorsed by ≥ 25% of the non-credible group and ≤ 10% of the credible group. The SIMS-MS demonstrated good differentiability between groups (AUC = .87), with good sensitivity (.53) and excellent specificity (.96) at a cutoff of > 1 (i.e., ≥ 2), and it did not significantly correlate with depressive (r = .21) nor anxiety (r = .23) symptoms, suggesting appropriate discriminant validity from face valid psychiatric symptoms. The SIMS-MS showed preliminary clinical utility at identifying individuals with MS who demonstrate non-credible cognitive performance on standalone PVTs. Implications, limitations, and directions for further inquiry, including validation against other measures of both performance and symptom validity, were discussed.