Assessment of regional glucose metabolism by [18F]fluorodeoxyglucose position emission tomography ([18F]FDG PET) serves as a biomarker for differential diagnosis of dementia. Conversely, depressive cognitive impairment shows no abnormalities on cerebral [18F]FDG PET.

This study validates the diagnostic value of [18F]FDG PET in addition to clinical diagnosis in a real-life gerontopsychiatric clinical population.

Ninety-eight consecutive patients with depression and cognitive impairment were included. Baseline clinical diagnoses were independently established before and after disclosure of [18F]FDG PET, and dichotomised into neurodegenerative or non-neurodegenerative diseases (level 1). Subsequently, neurodegenerative cases were allocated to diagnostic subgroups (Alzheimer’s disease, Lewy body diseases, frontotemporal lobar degeneration, neurodegenerative other; level 2). An interdisciplinary, biomarker-supported consensus diagnosis after a median follow-up of 6.6 month after [18F]FDG PET served as reference. Changes of clinical diagnoses and diagnostic accuracy were assessed.

After disclosure of [18F]FDG PET, level-1 clinical diagnoses changed in 23% (95% CI 16–33%) of cases, improving the diagnostic accuracy from 72% (95% CI 62–81%) to 92% (95% CI 84–96%) (P < 0.001). [18F]FDG PET was of particular value for exclusion of neurodegenerative disease. Concerning level-2 decisions, the clinical diagnoses changed in 30% (95% CI 21–40%) of cases, increasing its accuracy from 64% (95% CI 54–74%) to 85% (95% CI 76–91%) (P < 0.001). A major fraction of incorrect level-2 diagnoses comprised Alzheimer’s disease misdiagnosed as Lewy body diseases.

[18F]FDG PET provides a significant incremental diagnostic value beyond the clinical diagnosis in depressive cognitive impairment. Thus, [18F]FDG PET should be considered in the diagnostic work-up of patients with mental disorders and cognitive impairment.