Dear Dr. Rishabh Tomar

The Index Value is calculated by dividing the sample OD value by the cut-off-OD.

So, Index Value = OD sample / OD cut-off

Let me explain with an example.

Suppose an ELISA for a given test, the Index Values are as follows:

Negative < 1.0

Borderline 1.0 – 1.2

Positive: ≥ 1.2

For the OD cut-off of 0.420, let us consider the following sample OD values.

1. Sample OD: 0.110

2. Sample OD: 0.420

3. Sample OD: 1.220

Therefore, the index values for the above sample ODs would be as follows.

Sample 1: 0.261

Sample 2: 1.0

Sample 3: 2.904

The interpretation would be as follows:

Sample 1: negative

Sample 2: borderline

Sample 3: positive

Regards

Malcolm Nobre