ABtest40 and ABtest42 accuracy for plasma samples of 5 different concentrations. Each point represents the relative error (RE, %) of each of the measurements made. The dotted lines indicate the limit of compliance established by the validation guidelines (± 20 %), although in the case of the sample at the lower limit of quantification (LLOQ) ± 25 % is acceptable.
The table shows the lower limit of quantification (LLOQ) determined in accordance with EMA and FDA recommendations. These levels are well below the average levels detected in healthy people and patients with AD.
The chart shows the specificity of Aβ40 and Aβ42 determinations characterised by the almost complete absence of cross-reactivity with other peptides similar to each of the target peptides.
Plasma Aβ42/Aβ40 ratio (TP42/40) values measured by ABtest-IA correlate directly with cerebrospinal fluid Aβ42 levels and inversely with cortical amyloid burden measured by PET scanning. (https://pubmed.ncbi.nlm.nih.gov/31787105)
Plasma Aβ42/Aβ40 ratio predicts brain amyloid burden and can discriminate normal amyloid PET-positive vs. negative individuals with an area under the curve (AUC) of 0.896 among people without symptoms of cognitive impairment. (https://pubmed.ncbi.nlm.nih.gov/32179698).
The Aβ42/Aβ40 ratio measured by ABtest-IA is lower in individuals with cortical amyloid burden above the normal level in Amyloid-PET (Aβ+) than in individuals with a burden below the threshold (Aβ-) ( https://pubmed.ncbi.nlm.nih.gov/32179698).
If recruitment is carried out directly based on PET-amyloid level, 500 scans would be needed whereas if pre-screening of participants on the basis of 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 (TP42 and TP40).
