CSF Amyloid Beta 40 Unit Converter
CSF Amyloid Beta 40 Unit Converter
Convert CSF Aβ40 between pg/mL, ng/L, µg/L, pmol/L and nmol/L — the denominator of the Aβ42/Aβ40 ratio, and 4.08% lighter than Aβ42, which is why the molar ratio is not the reported one.
CSF Amyloid Beta 40 converter
Mass ⇄ molarCSF Aβ40 of 10,000 pg/mL, an illustrative value, with an Aβ42 of 700 pg/mL
The conversion, and the 4% that matters
pg/mL = pmol/L ÷ 0.230954
ng/L = pg/mL × 1 — the same quantity written two ways
molar Aβ42/Aβ40 = mass Aβ42/Aβ40 × 0.95919
- 4,329.87
- the molar mass of Aβ40, C194H295N53O58S. The same chain as Aβ42 without its C-terminal isoleucine and alanine, and 4.08% lighter. CAS 131438-79-4; the formula sum from standard atomic weights gives 4,329.867 against the vendor figure of 4,329.82
- 0.95919
- the ratio of the two masses, 4,329.87 ÷ 4,514.1. Because Aβ42 is the heavier peptide, a given mass of it is fewer moles, so the molar Aβ42/Aβ40 ratio comes out 4.08% BELOW the mass ratio the laboratory printed
- why that is a trap
- a reader who converts both peptides to pmol/L and re-divides gets a smaller ratio than the report — moving a result in the amyloid-positive direction, and potentially across the 0.073 threshold
Worked example
CSF Aβ40 of 10,000 pg/mL, an illustrative value, with an Aβ42 of 700 pg/mL
10,000 × 0.230954 = 2,309.5 pmol/L
10,000 pg/mL = 10,000 ng/L = 10.000 µg/L
Mass ratio Aβ42/Aβ40 = 700 ÷ 10,000 = 0.0700 — the number a laboratory reports
Molar ratio = (700 × 0.221528) ÷ 2,309.5 = 0.0671, which is 4.08% lower and on the other side of 0.0725
The two peptides, side by side
| Aβ40 | Aβ42 | |
|---|---|---|
| Residues | 40 | 42 |
| Formula | C194H295N53O58S | C203H311N55O60S |
| Molar mass | 4,329.87 Da | 4,514.1 Da |
| pg/mL to pmol/L | × 0.230954 | × 0.221528 |
| Typical CSF level | roughly ten times Aβ42 | see the Aβ42 converter |
| Aggregates into plaque | much less readily | yes — this is the species that deposits |
Why Aβ40 has no reference interval of its own
| Question | Answer |
|---|---|
| Does Mayo publish an Aβ40 interval? | No. Its Elecsys-based ADEVL panel reports Aβ42, t-tau, p-tau181 and the p-tau/Aβ42 ratio, and no Aβ40 |
| Does the FDA-authorised Lumipulse assay? | It reports the ratio, 0.001 to 1.000, not an Aβ40 interval |
| So what is Aβ40 for? | Normalising Aβ42 for individual amyloid production and for pre-analytical loss — see the ratio calculator |
| How much does it vary between people? | Severalfold, which is the whole reason the ratio outperforms Aβ42 alone |
| Is the Aβ40 value transferable between assays? | No. One published comparison found Lumipulse reads about twice the MSD V-PLEX value for the same sample |
Aβ40 as a denominator, and the molar trap in the ratio
Amyloid precursor protein is cleaved at several positions, and the two products that matter clinically are Aβ40 and Aβ42 — the same chain, differing only in whether cleavage left two extra residues on the C-terminus. Aβ40 is produced in far greater quantity and aggregates far less readily, so it stays largely in solution while Aβ42 is drawn into plaque. That difference in behaviour, from a difference of one dipeptide, is what the ratio exploits.
How much total amyloid a person produces varies severalfold between individuals, and that variation enters the Aβ42 concentration directly. Someone with a constitutively low amyloid output can have a low Aβ42 and no plaque at all; someone with a high output can have substantial plaque and an Aβ42 that still looks acceptable. Dividing by Aβ40 removes most of that, and it also removes most of the pre-analytical loss, because both peptides adsorb to tube walls to a similar degree. This is why the ratio rather than Aβ42 is a Core 1 biomarker in the 2024 Alzheimer’s Association criteria, and why the only FDA-authorised CSF amyloid assay reports the ratio as its result.
The trap this page exists for is in the arithmetic of that ratio. The figure laboratories report is a ratio of two mass concentrations, both in pg/mL. Because Aβ42 is the heavier peptide by 184.2 Da, a given mass of it contains fewer moles, so converting both peptides to pmol/L and re-dividing produces a ratio 4.08% lower than the one printed on the report — in the amyloid-positive direction. A reported ratio of 0.0750, comfortably negative on the Lumipulse scale, becomes 0.0719 in molar terms and lands inside the indeterminate band. The two numbers are both correct and they are not the same quantity, so a threshold borrowed for one must not be applied to the other.
There is no reference interval for Aβ40 on this page because no laboratory read for this batch publishes one. That is not an omission; it reflects the fact that Aβ40 is a denominator rather than a result. What it does need is to be in the right unit, and pg/mL and ng/L being identical is the one thing most likely to be assumed rather than checked.
Frequently asked questions
How do I convert CSF Aβ40 from pg/mL to pmol/L?
Multiply by 0.230954, from Aβ40’s molar mass of 4,329.87 g/mol (C194H295N53O58S). As with Aβ42, pg/mL and ng/L are numerically identical, so the molar column is the only conversion that changes the number.
Is the Aβ42/Aβ40 ratio a mass ratio or a molar ratio?
A mass ratio. Both peptides are reported in pg/mL and the ratio is taken directly, so every published threshold — including the FDA-authorised 0.058/0.073 pair — is a mass ratio. The molar ratio is lower by exactly 4,329.87 ÷ 4,514.1, or 4.08%, and the published thresholds do not apply to it.
What is a normal CSF Aβ40?
No laboratory read for this page publishes one, because Aβ40 is not interpreted on its own. It runs roughly ten times higher than Aβ42 and varies severalfold between individuals — variation that is a nuisance in an absolute result and the whole point of it as a denominator. One published comparison found Lumipulse reads about twice the MSD V-PLEX value for the same sample, so even the order of magnitude is assay-dependent.
Why is Aβ40 lighter than Aβ42 if they are the same protein?
They are the same chain cleaved at different points. Aβ40 ends at valine 40; Aβ42 carries two further residues, isoleucine and alanine. Removing that dipeptide takes C9H16N2O2 off the formula and 184.2 Da off the mass, which is 4.08% of 4,514.1.
Do I need Aβ40 at all if I have Aβ42?
For a single result read against your own laboratory’s cut-off, no. For anything compared between laboratories or over time, yes — the ratio is far less sensitive to assay calibration and to pre-analytical handling than Aβ42 alone, which is why the 2024 criteria list the ratio as Core 1 and the absolute Aβ42 concentration not at all.
Related calculators
References
- GenScript RP10004. beta-Amyloid (1-40) — C194H295N53O58S, 4329.82, CAS 131438-79-4, sequence DAEFRHDSGYEVHHQKLVFFAEDVGSNKGAIIGLMVGGVV. Product datasheet; accessed October 2026.
- Validation of the Lumipulse automated immunoassay for the measurement of core AD biomarkers in cerebrospinal fluid. Clin Chem Lab Med. 2021; doi:10.1515/cclm-2021-0651.
- Fujirebio Diagnostics. Lumipulse G beta-Amyloid Ratio (1-42/1-40) — cut-offs and clinical performance. Product information sheet, 2022.
- Jack CR Jr, Andrews JS, Beach TG, et al. Revised criteria for diagnosis and staging of Alzheimer’s disease: Alzheimer’s Association Workgroup. Alzheimers Dement. 2024. doi:10.1002/alz.13859.
- Commission on Isotopic Abundances and Atomic Weights, IUPAC. Standard Atomic Weights 2024. Accessed October 2026.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/