Table 1.

Comparison of different aspects relevant for implementation of Aβ-PET, CSF, and blood-based analysis methods

Topic	Aβ-PET imaging	CSF analysis	Blood analysis
Context of use (COU)	Used in clinical trials as an entry point for patient stratification with the aim to identify subjects with Aβ load in the brain [73]	More applications: early diagnosis, differential diagnosis, treatment follow-up	Multiple applications in the diagnostic or therapeutic area, including (pre-) clinical studies (PK/PD testing) Biomarkers can be disease-related or treatment-related, classified by the BEST biomarker category (Web-1)
Clinical Method comparison	Both methods have a high clinical accuracy to identify Aβ load in the brain. Concordance between results is high, although discrepant results have been published		Correlation with CSF for some proteins (e.g., NF-L, Aβ ratio), not for other proteins
			Comparison needs to be done with respect to the clinical utility or for a specific COU
			Possibility for use of blood protein panels in combination with other diagnostic tools (e.g., CSF) [21]
Clinical Change in function of disease process	Aβ-PET scan differences are visualized after changes in CSF biomarker profiles	Changes in concentrations of CSF proteins are detectable more than 20 years before cognitive symptoms appear	To be defined, as compared to Aβ-PET imaging or CSF analysis
			Comparison is dependent on the COU
Clinical trials	Used for patient stratification	Exploratory biomarker	Not integrated in trials yet
Analytes_number	One hallmark. Different outcome in function of the selected tracer	Several relevant proteins can be included in the panel (e.g., tau, Aβ, NF-L, …)	A huge number of proteins can be included in the panel
		Most proteins are detectable with classical technology platforms	Concentrations are lower than in CSF (for most of the analytes), but technologies allow low limit of detection
Cost	High cost, limiting repeated measures	Limited cost/sample	Low cost/sample
		Repeated sampling is possible, but more difficult	Repeated sampling is possible
Safety	Exposure to radiation. Problems with claustrophobia for demented subject	Lumbar Puncture is perceived as invasive and with a risk of 1–5% for severe headache problems [59]	Safe, routine procedure
Instrumentation	PET scanners: not available worldwide Cyclotrons: tracers need to be on-site within 1 h of preparation	Random-access immunoassay analyzers have been qualified for accurate and reliable quantification of CSF proteins [9]	Point-of-care test systems for multiple protein analysis will become available soon. Random-access analyzers are already available in many labs
	Technology is only available in specialized centers
Standardization Training	It needs to be performed independent of a patient’s clinical feature. Only by readers who successfully complete a special training program	CSF-Aβ1–42 data can be harmonized between technology platforms using CSF-based certified reference materials [32] Efforts are ongoing for a worldwide implementation of a SOP for collection and storage of CSF. Run-validation criteria can be established easily. Each lab technician, skilled in the art, can perform these immunoassays	No special training required, except availability of technicians, skilled in the art to perform immunoassays
Data interpretation	Visual interpretation or software. Flashy outcome	Reader and curve fit algorithms. Objective outcome. Interpretation is more difficult. No universal cutoff value available
Regulatory	The FDA approved Aβ-PET imaging by visual interpretation (‘not software’) to identify subjects with ongoing amyloidopathy (= exclusion criterium) [73]	EMA qualified the combined use of CSF Aβ and tau proteins as a tool for patient enrollment in clinical trials (Web-2) A letter of support from the FDA encourages their use in clinical trials (Web-3). The method must be used in adjunct to other diagnostic evaluations. Algorithms based on CSF proteins, together with imaging methods, were integrated into revised criteria for AD diagnosis [18] and research proposals for the amyloid/tau/neurodegeneration (A/T/N) classification of AD pathological processes [27]	No qualification proposals submitted
			No regulatory approved assay formats

Website references: Web-1 https://www.ncbi.nlm.nih.gov/books/NBK326791/; Web-2 https://www.ema.europa.eu/en/documents/regulatory-procedural-guideline/qualification-opinion-novel-methodologies-predementia-stage-alzheimers-disease-cerebrospinal-fluid_en.pdf; Web-3 https://www.fda.gov/drugs/cder-biomarker-qualification-program/letter-support-los-initiative

Aβ β-amyloid, COU context of use, CSF cerebrospinal fluid, EMA the European Medicine Agency, FDA The Food and Drug Administration, NF-L neurofilament light, PET positron emission tomography, SOP standard operating procedure