ELISA Titer Calculator
Analyze ELISA results by calculating the sample-to-cutoff (S/CO) ratio and estimating antibody titer from optical density (OD) readings.
ELISA Titer Calculator
intermediateCalculate sample-to-cutoff ratio and estimate antibody titer
Formula
S/CO = Sample OD / Cutoff OD
How It Works
ELISA (Enzyme-Linked Immunosorbent Assay) uses optical density measurements to quantify antibodies or antigens. The cutoff OD is the threshold value that separates positive from negative results.
S/CO Ratio = Sample OD / Cutoff OD
Estimated Titer = S/CO × Dilution Factor
Interpretation: S/CO ≥ 1.0 = Positive; S/CO < 1.0 = Negative. The titer estimate indicates the approximate endpoint dilution of the sample.
Example
A serum sample diluted 1:100 gives OD = 1.2. The cutoff is OD = 0.4:
S/CO = 1.2 / 0.4 = 3.00 (Strong Positive)
Estimated Titer = 3.00 × 100 = 300
The sample is strongly positive with an estimated titer of 1:300.
Frequently Asked Questions
How is the cutoff OD determined?
The cutoff is typically calculated from negative control samples: Cutoff = Mean(negative ODs) + 3×Standard Deviation, or a fixed multiplier of the negative control mean (e.g., 2.1× for some commercial kits).
What is a gray zone in ELISA?
Many assays define a "gray zone" of borderline results, typically S/CO between 0.9-1.1. Samples in this range should be retested or confirmed with a different method.
What does the titer estimate represent?
The titer is the highest dilution at which the sample still gives a positive result. A titer of 300 means the sample could be diluted 300-fold and still be positive.