Background: The Ascertain Dementia 8-item Questionnaire (AD8) is a validated informant-based interview for early dementia detection. Research suggests the utility of self-rated AD8 to identify milder dementia forms in research settings. This study compares the factor structure, reliability, and diagnostic performance between AD8-Self and AD8-Informant for early cognitive impairment (ECI) in a clinical setting. Methods: Five hundreds fifteen patient-informant dyads (43 cognitively intact and 472 ECI) from a tertiary memory clinic completed both self-reported and informant AD8. We conducted exploratory factor analysis to determine the factor structure, Cronbach’s alpha for internal consistency, and receiver operating characteristic (ROC) curve analysis for ECI, including a subgroup analysis for mild cognitive impairment (MCI). Results: The mean age and education of ECI participants were 75.61 years (range, 51–95) and 5.6 years (range, 0–20), respectively, and 72.6 years (range, 51–89) and 6.8 years (range, 0–16) in the MCI subgroup. Unlike AD8-Informant’s one-factor structure, AD8-Self had a two-factor structure corresponding to memory and non-memory domains. AD8-Self demonstrated lower reliability (Cronbach’s alpha: ECI 0.666 vs. 0.764; MCI 0.663 vs. 0.709). In ECI, AD8-Informant (cutoff score ≥3) showed better diagnostic performance (sensitivity 89%, specificity 79%) than AD8-Self (cutoff score ≥4; sensitivity 27.1%, specificity 95.3%) (AUC 0.915 vs. 0.593; p<0.001). Similar results were found in MCI (sensitivity 64.7% vs. 26.5%; specificity 79.1% vs. 95.3%; AUC 0.745 vs. 0.600; p=0.002). Conclusion AD8-Self has a distinct factor structure, lower reliability, and inferior diagnostic performance compared to AD8-Informant for ECI/MCI detection. Our result do not support AD8-Self as a standalone tool for detecting ECI or MCI.