Concordance Between Different Amyloid Immunoassays and Visual Amyloid Positron Emission Tomographic Assessment

Abstract

Importance: Visual assessment of amyloid positron emission tomographic (PET) images has been approved by regulatory authorities for clinical use. Several immunoassays have been developed to measure β-amyloid (Aβ) 42 in cerebrospinal fluid (CSF). The agreement between CSF Aβ42 measures from different immunoassays and visual PET readings may influence the use of CSF biomarkers and/or amyloid PET assessment in clinical practice and trials.

Objective: To determine the concordance between CSF Aβ42 levels measured using 5 different immunoassays and visual amyloid PET analysis.

Design, setting, and participants: The study included 262 patients with mild cognitive impairment or subjective cognitive decline from the Swedish BioFINDER (Biomarkers for Identifying Neurodegenerative Disorders Early and Reliably) cohort (recruited from September 1, 2010, through December 31, 2014) who had undergone flutemetamol F 18 ([18F]flutemetamol)-labeled PET. Levels of CSF Aβ42 were analyzed using the classic INNOTEST and the newer modified INNOTEST, fully automated Lumipulse (FL), EUROIMMUN (EI), and Meso Scale Discovery (MSD) assays. Concentrations of CSF Aβ were assessed using an antibody-independent mass spectrometry-based reference measurement procedure.

Main outcomes and measures: The concordance of CSF Aβ42 levels and Aβ42:Aβ40 and Aβ42:tau ratios with visual [18F]flutemetamol PET status.

Results: Of 262 participants (mean [SD] age, 70.9 [5.5] years), 108 were women (41.2%) and 154 were men (58.8%). The mass spectrometry-derived Aβ42 values showed higher correlations with the modified Aβ42-INNOTEST (r = 0.97), Aβ42-FL (r = 0.93), Aβ42-EI (r = 0.93), and Aβ42-MSD (r = 0.95) assays compared with the classic Aβ42-INNOTEST assay (r = 0.88; P ≤ .01). The signal in the classic Aβ42-INNOTEST assay was partly quenched by recombinant Aβ1-40 peptide. However, the classic Aβ42-INNOTEST assay showed better concordance with visual [18F]flutemetamol PET status (area under the receiver operating characteristic curve [AUC], 0.92) compared with the newer assays (AUCs, 0.87-0.89; P ≤ .01). The accuracies of the newer assays improved significantly when Aβ42:Aβ40 (AUCs, 0.93-0.95; P ≤ .01), Aβ42 to total tau (T-tau) (AUCs, 0.94; P ≤ .05), or Aβ42 to phosphorylated tau (P-tau) (AUCs, 0.94-0.95; P ≤ .001) ratios were used. A combination of the Aβ42:Aβ40 ratio and T-tau or P-tau level did not improve the accuracy compared with the ratio alone.

Conclusions and relevance: Concentrations of CSF Aβ42 derived from the new immunoassays (modified INNOTEST, FL, EI, and MSD) may correlate better with the antibody-independent mass spectrometry-based reference measurement procedure and may show improved agreement with visual [18F]flutemetamol PET assessment when using the Aβ42:Aβ40 or Aβ42:tau ratios. These findings suggest the benefit of implementing the CSF Aβ42:Aβ40 or Aβ42:tau ratios as a biomarker of amyloid deposition in clinical practice and trials.

Figure 1.. Cerebrospinal Fluid (CSF) Alzheimer Disease (AD) Biomarkers and Flutemetamol F 18–Labeled Positron Emission Tomography (PET)

A, Concordance between visual and quantitative amyloid PET analysis. Composite standardized uptake value ratio (SUVR) of [¹⁸F]flutemetamol data in patients who were classified as negative (n = 149) or positive (n = 113) for amyloid findings with visual PET assessment are shown. The dotted line represents a cutoff of greater than 1.42 SUVR. Data points indicate individuals. B-F, Correlation of CSF AD biomarkers with [¹⁸F]flutemetamol PET status according to visual analysis. Levels of CSF β-amyloid (Aβ) 42 were analyzed with the classic INNOTEST (INC), modified INNOTEST (INM), EUROIMMUN (EI) and Meso Scale Discovery (MSD) assays. To calculate the CSF Aβ42:Aβ40 ratio, we used Aβ40 kits from the assay vendors Fujirebio (INC and INM), EUROIMMUN (EI), and Meso Scale Discovery (MSD). Levels of CSF total tau (T-tau) and phosphorylated tau (P-tau) were measured with the EI and INNOTEST assays, respectively. Receiver operating characteristic (ROC) curves were generated for Aβ42 level (B-F), the Aβ42:Aβ40 ratio (C [INC] and D [INM, EI, and MSD]), the Aβ42:T-tau ratio (C [INC] and E [INM, EI, and MSD]), and the Aβ42:P-tau ratio (C [INC] and F [INM, EI, and MSD]) to determine their accuracy in differentiating Aβ-negative (n = 149) and Aβ-positive (n = 113) visual readings. AUC indicates area under the ROC curve.

Figure 2.. Frequency Plots of Cerebrospinal Fluid (CSF) β-Amyloid (Aβ) 42 Levels and Aβ42:Aβ40 and Aβ42 to Phosphorylated Tau (P-tau) Ratios

Histograms of frequency distribution for CSF Aβ42 levels and the Aβ42:Aβ40 and Aβ42:P-tau ratios across groups with Aβ-positive (n = 113) and Aβ-negative (n = 149) visual ratings. Veritical dashed lines indicate cutoff points associated with the Youden J index. EI indicates EUROIMMUN; INC, classic INNOTEST; INM, modified INNOTEST; and MSD, Meso Scale Discovery.

Figure 3.. Separation of Populations by Visual Positron Emission Tomography (PET) Findings

Separation of populations with positive and negative PET findings used cutoffs for cerebrospinal fluid (CSF) levels of β-amyloid (Aβ) 42 and the Aβ42:Aβ40 and Aβ42 to phosphorylated tau (P-tau) ratios. Scatterplots of CSF Aβ42 levels against Aβ40 and P-tau levels; Aβ42 was measured using the classic INNOTEST (INC), modified INNOTEST (INM), EUROIMMUN (EI), and Meso Scale Discovery (MSD) assays. Dotted lines indicate Youden J index cutoffs for CSF Aβ42 levels. Solid lines indicated Youden J index cutoffs for the CSF Aβ42:Aβ40 and Aβ42:P-tau ratios.