Neuropsychological interviews and neuroimaging techniques are traditional diagnostic methods for Alzheimer’s disease (AD). However, the development of blood-based biomarkers, such as Amyloid beta (Aβ), phosphorylated Tau (pTau), and their ratios, offers promising non-invasive alternatives for early AD detection. This study aimed to analyze the correlation between CSF and plasma biomarkers (Aβ40, Aβ42, Aβ42/Aβ40, pTau181) and evaluate their diagnostic performance in 51 patients with cognitive impairments. Biomarkers were analyzed in both plasma and CSF using an automated chemiluminescence enzyme immunoassay, Lumipulse (Fujirebio). The results showed significant positive correlations between CSF and plasma levels of Aβ42, the Aβ42/Aβ40 ratio, and pTau181, but not for Aβ40. Plasma Aβ42, pTau181, Aβ42/Aβ40 ratio, and pTau181/Aβ42 ratio demonstrated significant differences between patients A+ vs. A− classified based on CSF Amyloid status, as well as between those classified as A+T+ and A−T− according to both CSF Amyloid and Tau levels. Plasma pTau181, Aβ42/Aβ40, and pTau181/Aβ42 ratio showed high diagnostic accuracy in distinguishing A+ from A− (AUC = 0.93–0.95) and A+T+ from A−T− patients (AUC = 0.93–0.97). These findings suggest that plasma biomarkers can effectively differentiate between AD and other forms of dementia, and serve as a reliable, non-invasive tool for early detection and monitoring of Alzheimer’s disease.