Preliminary Validation of the FCD-Q8 Tool in Functional Cognitive Disorder and Early Alzheimer's Disease: A Biomarker-Verified Case-Control Study.

BACKGROUND: Differentiating functional cognitive disorder (FCD) from early Alzheimer's disease (eAD) through clinical interview alone is challenging and may lead to unnecessary testing. We aimed to assess the diagnostic accuracy of the Functional Cognitive Disorder Questionnaire-8 (FCD-Q8) and examine its psychometric properties. METHODS: In this cross-sectional, phase 1 case-control study at a tertiary memory clinic in Seville, Spain, 78 adults with memory complaints were recruited: 44 with biomarker-verified eAD (positive CSF or amyloid PET; GDS 3-4) and 34 with FCD (negative biomarkers and clinical internal inconsistency). A blinded neuropsychologist administered the FCD-Q8. The primary outcome was diagnostic accuracy (area under the ROC curve [AUC], sensitivity, and specificity). Secondary outcomes included internal consistency (Cronbach's α with 95% CI) and item characteristics using two-parameter logistic Item Response Theory (2PL IRT). RESULTS: Participants had a mean age of 67.5 years (SD ± 7.1); 59% were female. The FCD-Q8 demonstrated good diagnostic accuracy with an AUC of 0.87 (95% CI 0.78-0.95). At a cut-off ≥ 5, sensitivity was 76.5% and specificity 88.6%. Internal consistency was moderate (α = 0.61; 95% CI 0.47-0.70). 2PL IRT analysis showed that items related to checking behaviours were the most discriminative (difficulty 3.70), while questions on compound statements and premorbid self-appraisal were the least discriminative. Attending the clinical interview alone was the most difficult item (1.47). CONCLUSION: The FCD-Q8 demonstrated good diagnostic utility to distinguish FCD from eAD in biomarker-verified populations. These findings support progression to a subsequent prospective validation phase, which could further establish its clinical and research applicability.