I have learned that your question was answered in the study described below. But yes the 42-40 ratio does change to reflect a decay -clearing of the amyloid from brain to spinal canal, showing up in CSF. That said, i have a big question about a specific
Reference in study design description that I will try to get answered this week.

The research described in the comment refers to a long-term randomized clinical trial conducted by Orestes V. Forlenzaand colleagues at the University of São Paulo, Brazil. Specifically, the findings regarding the 30% increase in CSF Aβ42 levels after 36 months were published in the following article:
Forlenza, O. V., Radanovic, M., Talib, L. L., & Gattaz, W. F. (2019). "Clinical and biological effects of long-term lithium treatment in older adults with amnestic mild cognitive impairment: randomised clinical trial." The British Journal of Psychiatry, 215(5), 668–674
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Key Details of the Research:
Study Design: A single-center, randomized, double-blind, placebo-controlled trial involving 61 older adults with amnestic mild cognitive impairment (aMCI) 1.
Treatment: Participants received low-dose lithium carbonate aimed at subtherapeutic serum concentrations (0.25–0.5 mEq/L) for up to 4 years 1, 2.
Biomarker Results: While the placebo group showed stable or declining levels of CSF Aβ1-42, the lithium-treated group experienced a significant increase in these concentrations at the 36-month mark 1, 2.
Interpretation: This increase is interpreted as a sign of enhanced amyloid clearance from the brain parenchyma into the cerebrospinal fluid, potentially reflecting a disease-modifying effect that prevents the further accumulation of toxic amyloid plaques in the brain 1, 2.
This study is part of the broader "Lithium-MCI" trial series, which also includes earlier reports on the reduction of phosphorylated tau (P-tau) after 12 months of treatment 3 and a recent 13-year follow-up demonstrating sustained cognitive benefits in the lithium-treated cohort 2.