Objectives: With the development of new technologies capable of detecting low concentrations of Alzheimer's disease (AD) relevant biomarkers, the idea of a blood-based diagnosis of AD is nearing reality. This study aims to consider the evidence of total and phosphorylated tau as blood-based biomarkers for mild cognitive impairment (MCI) and AD when compared to healthy controls.
Methods: Studies published between 1 January 2012 and 1 May 2021 (Embase and MEDLINE databases) measuring plasma/serum levels of tau in AD, MCI, and control cohorts were screened for eligibility, including quality and bias assessment via a modified QUADAS. The meta-analyses comprised 48 studies assessing total tau (t-tau), tau phosphorylated at threonine 181 (p-tau181), and tau phosphorylated at threonine 217 (p-tau217), comparing the ratio of biomarker concentrations in MCI, AD, and cognitively unimpaired (CU) controls.
Results: Plasma/serum p-tau181 (mean effect size, 95% CI, 2.02 (1.76-2.27)) and t-tau (mean effect size, 95% CI, 1.77 (1.49-2.04)) were elevated in AD study participants compared to controls. Plasma/serum p-tau181 (mean effect size, 95% CI, 1.34 (1.20-1.49)) and t-tau (mean effect size, 95% CI, 1.47 (1.26-1.67)) were also elevated with moderate effect size in MCI study participants compared to controls. p-tau217 was also assessed, albeit in a small number of eligible studies, for AD vs. CU (mean effect size, 95% CI, 1.89 (1.86-1.92)) and for MCI vs. CU groups (mean effect size, 95% CI, 4.16 (3.61-4.71)).
Conclusions: This paper highlights the growing evidence that blood-based tau biomarkers have early diagnostic utility for Alzheimer's disease.
Registration: PROSPERO No. CRD42020209482.
Keywords: Alzheimer’s disease; biomarkers; blood; diagnostics; mild cognitive impairment; tau.