Reproducibility of Alzheimer's Disease Cerebrospinal Fluid-Biomarker Measurements under Clinical Routine Conditions

Abstract

Analysis of cerebrospinal fluid (CSF) is one of the key tools for the state-of-the-art differential diagnosis of dementias. Dementia due to Alzheimer's disease (AD) is characterized by elevated CSF levels of total Tau (tTau) and phospho-181-Tau (pTau) and low CSF amyloid-β42 (Aβ42). Discrepancies in the laboratory analysis of human materials are well known and much effort has been put into harmonization procedures. In this study, we measured CSF biomarkers of more than 100 patients obtained under clinical routine conditions in two different clinical laboratories. The CSF biomarker levels obtained from the two different sites were significantly correlated: R2 = 0.7129 (tTau, p < 0.001), 0.7914 (pTau, p < 0.001), 0.5078 (Aβ42, p < 0.001), 0.5739 (Aβ40, p < 0.001), and 0.4308 (Aβ42/40, p < 0.001). However, the diagnostic classifications of the Aβ42, tTau, and pTau levels of identical subjects into normal versus pathological range made by the two different sites showed substantial discrepancies (31.5%, 29.6%, and 25.0% discordant cases, respectively). Applying Aβ42/40, instead of CSF Aβ42 alone, lead to a reduction of the discordant cases to 16.8%. Our findings suggest that CSF Aβ42/40 can outperform Aβ42 as a biomarker for AD neuropathology, not only under well-controlled study conditions but also in real life clinical routine. Thus, we recommend the inclusion of Aβ42/40 as a CSF biomarker in the diagnostic procedure.

Keywords: Alzheimer’s disease; amyloid-β; biomarker; cerebrospinal fluid; clinical diagnostics.

Fig.1

Bland-Altman plots for each biomarker. The mean value of the two biomarker measurements was plotted on the X-axis. On the Y-axis the difference between both centers (center 1 – center 2) was pictured. This presentation facilitates the direct comparison between two different centers. While no systematic differences were observed regarding the measurements for tTau, pTau, and Aβ₄₀, the measured Aβ₄₂ levels and, as a result Aβ_42/40, were generally higher in center 1.

Fig.2

Measured concentrations of tTau, pTau, Aβ₄₀, and Aβ₄₂ in both centers presented as scatter blots with the corresponding cut-off points. For Aβ_42/40, both centers had the same cut-off points (0.05 Aβ₄₂/Aβ₄₀). Interestingly, in center 2, two different subgroups, most likely AD patients and controls, can be seen, which are separated by the cut-off of 0.05 (E). In contrast, no such separations can be seen in center 1. Significant differences are indicated as **p < 0.01 and ***p < 0.001.

Fig.3

Correlation between CSF-biomarker measurements in the two different centers. Cut-off points of each center are indicated as red horizontal and vertical lines. Patients with discordantly interpreted biomarkers can be found in the upper left square for tTau (A) and pTau (B), whereas for Aβ, discordant biomarkers are mainly in the lower right square (C: Aβ₄₂ and D: Aβ_42/40). All biomarkers correlated between both centers: R² = 0.7129, p < 0.001 (tTau); R² = 0.7914, p < 0.001 (pTau); R² = 0.5078, p < 0.001 (Aβ₄₂); R² = 0.5739, p < 0.001 (Aβ₄₀); R² = 0.4308, p < 0.001 (Aβ_42/40).