Antibody Titre Calculator
The titre of a serum from a serial dilution — the last dilution that still gives a positive result — with its log value, the fold rize against a previous sample, and whether that rize counts as seroconversion.
Serum is diluted in steps — usually twofold, from a starting dilution of 1 in 10 or so — and each well is tested.
How the antibody titre calculator works
Serum is diluted in steps — usually twofold, from a starting dilution of 1 in 10 or so — and each well is tested. The titre is the reciprocal of the last dilution that is still positive: if well 7 of a twofold series starting at 1:10 is the last positive, the titre is 10 × 2⁶ = 640. Titres are compared on a log scale, and a fourfold rize between two samples taken weeks apart is the conventional sign of a recent infection or a successful vaccine.
Formula: titre = start × factor^(n − 1); fold rise = titre / previous; seroconversion: rise ≥ 4
Worked examples
| Inputs | Titre | Note |
|---|---|---|
| Well 7 of a twofold series from 1:10 | 1:640 | 1:640 — an eightfold rize |
| A fourfold series | 1:640 | 1:640 as well |
| Positive to the last well | 1:20480 (the last well of the series — the true titre may be higher) | may be higher still |
FAQFrequently asked questions
What is an antibody titre?
The greatest dilution of a serum that still gives a positive test — a measure of how much antibody it contains. It is reported as the reciprocal: a titre of 640 means positive at 1 in 640.
Why twofold dilutions?
Convention and convenience: each well halves the previous. The price is that a titre is only known to within a factor of two, which is why single titres are compared on a log scale.
What is seroconversion?
A rize of at least fourfold between two samples — two dilution steps — which is beyond the assay's noize and indicates a recent immune response.
What if every well is positive?
The titre is at least the last dilution and may be much higher. Re-run from a higher starting dilution to find the endpoint.
Does a higher titre mean more protection?
Usually, for the same assay and pathogen, but the protective level differs by disease and by test. Titres are comparable within a laboratory's method, not across methods.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- National Human Genome Research Institute — the US genomics research institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.