Background Cerebrospinal fluid (CSF) analysis for detecting amyloid positivity may be as reliable as positron emission tomography (PET). We evaluated the performance of the amyloid beta (Aβ)42/40 ratio for predicting amyloid positivity by PET, compared with Aβ42 alone, and phosphorylated tau 181 (pTau181)/Aβ42 and total tau (tTau)/Aβ42 ratios, using fully automated CSF immunoassays (Roche Diagnostics International Ltd, Rotkreuz, Switzerland) in a heterogeneous cohort of patients with a range of cognitive disorders reflecting the typical population of a memory clinic. Methods CSF samples from 103 patients with known amyloid PET status (PET positive = 54; PET negative = 49) were retrospectively selected from one site in Germany; 71 patients were undergoing treatment for mild cognitive impairment (n = 44) or mild-to-moderate dementia (n = 27) due to Alzheimer’s disease (AD), and 32 patients were undergoing treatment for non-AD-related cognitive disorders. Aβ42, pTau181, and tTau concentrations were measured in CSF samples using the respective Elecsys® CSF immunoassays modified for use on the cobas e 411 analyzer; Aβ40 concentrations were measured using a non-commercially available robust prototype assay. Sensitivities/specificities for amyloid positivity cut-offs (Youden-derived and pre-defined) were calculated, and receiver operating characteristic analyses determined area under the curve (AUC) versus amyloid PET status. Limitations include a small sample size, use of a pre-analytical protocol not in accordance with the Elecsys CSF immunoassay method sheets, and the lack of a pre-defined cut-off for Aβ42/40. Results Point estimates for sensitivity and specificity of CSF biomarkers and biomarker ratios versus amyloid PET were 0.93 and 0.57 for Aβ42, 0.96 and 0.69 for pTau181/Aβ42, 0.92 and 0.69 for tTau/Aβ42, and 0.94 and 0.82 for Aβ42/40. For AUCs, point estimates (95% confidence intervals) versus amyloid PET were 0.78 (0.68−0.88) for Aβ42, 0.88 (0.81−0.95) for pTau181/Aβ42, 0.87 (0.80−0.95) for tTau/Aβ42, and 0.90 (0.83−0.97) for Aβ42/40. Conclusions CSF Aβ42/40 ratio can predict PET amyloid positivity with high accuracy in patients with a range of cognitive disorders when evaluating Aβ pathology independent of tau and neurodegeneration for research purposes. The performance of Aβ42/40 was comparable with pTau181/Aβ42 and tTau/Aβ42 used in clinical practice and better than Aβ42 alone.
Background pTau181/Aβ42 (A+/T+ as per A/T/(N) classification) and tTau/Aβ42 (A+/(N)+) ratios using fully automated Elecsys® cerebrospinal‐fluid (CSF) immunoassays have been shown to perform well in distinguishing amyloid‐positivity (A+) by positron emission tomography (PET) among patients with subjective and mild to severe cognitive impairment. Higher performance has also been demonstrated for the CSF Aβ42/40 ratio compared to CSF amyloid 42 (Aβ42) alone. We aimed to further evaluate the performance of the Elecsys® Aβ42/40 ratio in a clinical cohort. Methods Frozen (‐80°C) CSF samples from 103 patients treated in the Centre for Cognitive Disorders, Technical University of Munich were selected based on availability of visual amyloid‐PET status (n=54 PET+; 49 PET‐) determined during standard treatment around the time of CSF collection. In total, 71 patients were undergoing treatment for mild cognitive impairment (MCI) (n=44) or mild to moderate dementia (n=27) due to Alzheimer’s disease (AD) and 32 patients for non‐AD related fronto‐temporal lobar degeneration (n=17), MCI (n=8), depression (n=5), alcohol dependence (n=1), or subjective cognitive impairment (SCD) (n=1). CSF was measured using Elecsys® assays. CSF status for Aβ42 , pTau181/Aβ42 , and tTau/Aβ42 CSF (positive/negative) was determined based on pre‐defined biomarker cutoffs previously established using a different pre‐analytical protocol in MCI/SCD patients with suspected AD (CSF positive if Aβ42<1000 pg/ml, pTau181/Aβ42>0.024, and tTau/Aβ42>0.028). Sensitivity and specificity were calculated for CSF‐cutoff‐based status and ROC analysis was performed to determine AUC each versus visual amyloid‐PET status. Results Pre‐defined biomarker thresholds were considered off‐label due to deviation from recommended sample collection protocol and storage parameters. Point estimates for sensitivity and specificity for each biomarker were; CSF Aβ42 alone (sensitivity = 0.93, specificity = 0.57), pTau181/Aβ42 (0.96, 0.69), and tTau/Aβ42 (0.92, 0.69). For AUC point estimates (95% confidence interval) were; CSF Aβ42 alone [0.78 (0.68‐0.88)], pTau181/Aβ42 [0.88 (0.81‐0.95)], tTau/Aβ42 [0.87(0.80‐0.95)], and Aβ42/40 [0.90 (0.83‐0.95)]. Conclusion Fully automated Elecsys® cerebrospinal fluid (CSF) immunoassay predicted amyloid positivity with high accuracy. In this study, the solely A+ marker CSF Aβ42/40 ratio outperformed Aβ42 alone and performed similarly to Tau ratios. These results support those of previous studies showing high performance for the non‐Tau marker CSF Aβ42/40.
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