I am working with NOVATECH ELISA KIT for Brucella. In the leaflet, it is mentioned that it is a qualitative assay with a cutoff starting at 11. When I enrolled the assay through the DS2 automatic ELISA machine, the layout results showed values as numbers, not only positive and negative. What does it mean? Are those numbers the real titre value? Are they the real concentrations of measured antibodies? If yes, why has the company not highlighted the kit as a quantitative ELISA?
Hi,
They are not the real concentrations. The numbers are for you to evaluate if your samples are positive or negative. If the cut-off value is 11, that means that your samples that have a value of 11 and above are positive.
A more correct way to determine if positive or negative is: Above the mean of your negative control samples by 3 standard deviations consider positive.
Article Calculation of the ELISA’s cut-off based on the change-point...
I hope that helps
Dear Shimos,
in indirect ELISA, the intensity of color (measured as optical density) usually reflects antibody concentration. i.e intensity of the color is directly proportional to the antibody concentration in patients samples. more color indicate more antibody.
The '''cutoff value' is used to determine if the sample is positive (= or greater than the cutoff) or negative (lower than cutoff).
In your case the cutoff is 11 so any sample score 11 or above will be positive for antibody.
Hope that is useful
Dear Shimos A. Alshareef
ELISA may be run in a qualitative or quantitative format.
Qualitative results provide a simple positive or negative result (yes or no) for a sample. The cut-off between positive and negative is determined by the analyst and may be statistical.
For diagnostic tests that produce a numerical result, the point above which test results are classified as positive is called the cut-off. These are usually established by testing a large number of infected and non-infected animals and selecting the value that maximizes the sensitivity and specificity of the test. For diagnostic kits the cut-off is established by the kit manufacturer. You may want to refer to the article attached below.
https://inis.iaea.org/collection/NCLCollectionStore/_Public/29/067/29067671.pdf
In quantitative ELISA, the optical density of the sample is compared to a standard curve, which is typically a serial dilution of a known concentration solution of the target molecule. For instance, if a test sample shows an absorbance of 1.0, the point on the standard curve that gives an OD = 1.0 will be of the same analyte concentration as the sample.
So in this case, the samples showing values 11 and above are considered positive.
Best.
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