If recruitment is carried out directly based on PET-amyloid level, 500 scans would be needed, whereas if pre-screening of participants based on their plasma Aβ42/Aβ40 levels is implemented, the number of PET scans would be reduced to 208 to confirm the plasma test result.

This two-step screening strategy would reduce the screening failure rate from 70% to ~28%, making participation in clinical trials less demanding for individuals and substantially reducing the length of the screening period and associated costs. In addition, ABtest-IA allows differentiated determination of the levels of free Aβ40 and Aβ42 fraction in plasma (FP40 and FP42) as well as their total plasma levels (TP40 and TP42).

The free fraction (FP40 and FP42) obtained by our immunoassay is determined from an undiluted sample and represents the fraction of these peptides that is directly available in the sample, free of interactions with other plasma components that prevent their immuno-detection.

The total levels (TP40 and TP42) are obtained from an aliquot of the same sample, diluted in a proprietary buffer so that, together with the free one, the fraction of these peptides that was initially sequestered for the assay due to their interactions with other plasma components is also measured. Overall, the free fraction represents only about 40% of the total peptide quantified.

Our results indicate that, in most cases, determining the TP42/40 ratio is sufficient for practical purposes regarding disease detection or treatment monitoring. However, in certain research projects the additional assessment of the free fraction of both peptides (FP40, FP42) could provide information about the effect that the plasma matrix of each individual might be exerting on the metabolism and blood Aβ-peptide levels.

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