Cerebrospinal fluid ratio of phosphorylated tau protein and beta amyloid predicts amyloid PET positivity

Aim: Current diagnostic criteria for Alzheimer's dis ease shift the focus from clinical findings to metabolic biomarkers. Most widely used metabolic biomarkers in the Czech Republic are cerebrospinal fluid levels of beta amyloid 1-42, total tau, and phosphorylated tau 181, as well as amyloid PET. Our aim was to investigate the concordance of amyloid PET and cerebrospinal fluid biomarkers in clinical settings and propose a way of interpreting cerebrospinal fluid biomarker results in order to better predict the amyloid PET status in contradictory cases. Methods: A total of 103 patients underwent neuropsychological assessment, brain MRI, visually evaluated flutemetamol amyloid PET and examination of biomarkers in the cerebrospinal fluid. Concordance of amyloid PET and cerebrospinal fluid biomarkers was compared and subsequent optimal cut-off for the beta amyloid 1-42 and phosphorylated tau protein ratio was calculated. Results: Concordance between amyloid PET and beta amyloid 1-42 was 79%, followed by phosphorylated tau protein (72%). In 40% of cases, the results of cerebrospinal fluid biomarkers was contradictory. Beta amyloid 1-42 and phosphorylated tau protein ratio was found to best discriminate between amyloid PET positive and negative patients (AUC = 0.938, 95% CI = 0.890-0.986; P < 0.001). Ratio of 9.47 provided 91.0% sensitivity and 91.3% specificity. Conclusion: Both methods show good diagnostic concordance. In case of contradictory cerebrospinal fluid biomarkers, we encourage using the beta amyloid 1-42 and phosphorylated tau protein ratio.