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Background: Preclinical studies have found differential effects of isoflurane and propofol on the Alzheimer's disease (AD)- associated markers tau, phosphorylated tau (p-tau) and amyloid-β (Aβ). Objective: We asked whether isoflurane and propofol have differential effects on the tau/Aβ ratio (the primary outcome), and individual AD biomarkers. We also examined whether genetic/intraoperative factors influenced perioperative changes in AD biomarkers. Methods: Patients undergoing neurosurgical/otolaryngology procedures requiring lumbar cerebrospinal fluid (CSF) drain placement were prospectively randomized to receive isoflurane (n = 21) or propofol (n = 18) for anesthetic maintenance. We measured perioperative CSF sample AD markers, performed genotyping assays, and examined intraoperative data from the electronic anesthesia record. A repeated measures ANOVA was used to examine changes in AD markers by anesthetic type over time. Results: The CSF tau/A ratio did not differ between isoflurane- versus propofol-treated patients (p = 1.000). CSF tau/Aβ ratio and tau levels increased 10 and 24 h after drain placement (p = 2.002×10-6 and p = 1.985×10--6, respectively), mean CSF p-tau levels decreased (p = 0.005), and Aβ levels did not change (p = 0.152). There was no interaction between anesthetic treatment and time for any of these biomarkers. None of the examined genetic polymorphisms, including ApoE4, were associated with tau increase (n = 9 polymorphisms, p > 0.05 for all associations). Conclusion: Neurosurgery/otolaryngology procedures are associated with an increase in the CSF tau/A ratio, and this increase was not influenced by anesthetic type. The increased CSF tau/A ratio was largely driven by increases in tau levels. Futurework should determine the functional/prognostic significance of these perioperative CSF tau elevations. [ABSTRACT FROM AUTHOR]