Comparison of Aβ1-42/Aβ1-40 ratio with other ratios

CSF Aβ1-42/Aβ1-40 is a tool thought to normalize atypical amyloid levels, whereas other ratios might be seen more as interpretation tools that allow the comparison of different biomarker results.

Whereas other well-studied ratios including p-tau/Aβ1-42 and t-tau/Aβ1-42 combine different pathological features, the Aβ1-42/Aβ1-40 ratio is a combination of two markers reflecting the same pathology. When other biomarkers such as t-tau or p-tau are included, it must be taken into consideration that Tau and Aβ may not have a linear relationship over the course of the entire disease as they are parts of different pathological pathways. Therefore, every use of a ratio combining different pathologies needs to be evaluated and assessed critically concerning its value and result at different states of the disease and lifespan of the patient.

Tau / amyloid ratios

Among the scientific community it is generally widely accepted that combinations of different biomarkers outperform the use of individual biomarkers alone.

In a recent study by Schindler et al., both tau ratios, t-tau/Aβ1-42 and p-tau/Aβ1-42, were analyzed alongside Aβ1-42/Aβ1-40. The concordance of each ratio with PIB PET was compared to the individual biomarkers. The researchers concluded that ratios of CSF biomarkers that included Aβ42 (t-Tau/Aβ42, p-Tau/Aβ42 and Aβ42/Aβ40) best distinguished PIB-positive from PIB-negative individuals. Notably, they found that in cases of partial discordance between CSF biomarker ratios and PIB PET, Aβ42/Aβ40 was typically the positive ratio in PIB-negative cases and was the sole positive ratio in two PIB-positive cases.¹⁸ These findings suggest that abnormal Aβ42/Aβ40 may be the earliest indicator of amyloid brain pathology, potentially reflecting stage one of preclinical AD (amyloid deposition but no abnormalities in t-Tau or p-Tau).¹⁹ Nevertheless, since there are other pathologies (e.g. Tau), and other ratios that can combine different pathological models to evaluate AD risk, all ratios must be used in strict context, according to a well-defined clinical question.

An area of important consideration is whether individual biomarker levels and the respective ratios vary across different ethnicities. Recent publications have compared the levels of CSF biomarkers in Black Americans to those in non-Hispanic White Americans. Across three studies including 390 Black Americans, p-tau and t-tau levels were significantly lower in Black Americans than non-Hispanic Whites.^20–22 In Morris et al. this difference was most significant in Black Americans with an APOE4 allele, while in Howell et al. lower t-tau and p-tau levels in Black Americans were independent from APOE4 status.

When looking at amyloid proteins, Aβ42 levels were comparable between races. Howell et al. further examined the Aβ42/Aβ40 ratio and found similar levels between Black Americans and non-Hispanic Whites, whereas all three studies examining the p-tau/Aβ42 and t-tau/Aβ42 ratios found significant differences between races. This difference will be an important consideration for utilizing the tau ratios to determine amyloid pathology in Black Americans.

Other amyloid / amyloid ratios

From the limited available research, the Aβ1-42/Aβ1-40 and Aβ1-42/Aβ1-38 ratio perform similarly, each displaying high concordance with PET. Of note, Aβ1-38 may be increased in neuroinflammatory diseases.1 However, to date the assays for Aβ1-40 have been preferentially utilized over Aβ1-38 leading to less data available on Aβ1-38.

Influence of automation on the Aβ1- 42/Aβ1-40 ratio and its use

Automation is an important step in the direction of more standardization as it limits the number of manual handling steps. By minimizing handling, the risk of mistakes due to manual manipulations of the samples or the assay is reduced.

As it is generally seen as important to evaluate both parameters in the same run, the development of an automated system that allows testing of both Aβ1-42 and Aβ1-40 in parallel eliminates not only manual interactions but also run-to-run variability. This capability represents a big step towards standardization. Automation of the analysis of Aβ1-42 and Aβ1-40 with individual test results and the possibility of a ratio calculation as automated output could improve and simplify the use of the ratio and could allow more clinicians to use this tool to normalize their Aβ1-42 results.

In addition, false positives or other mistakes can be observed due to complex preanalytical conditions. If an automated system is used, it could be – due to economical consideration and lab organization - easier to test Aβ1-42 and Aβ1-40 simultaneously and in routine. This would further increase the level of reliability of the results as it combines the higher level of standardization (less manual interaction, standardized protocol, etc.) linked to automated testing with a higher degree of reliability of the Aβ1- 42/Aβ1-40 ratio in comparison with one parameter alone.