If the detection range is in ng/ml but the reference range is in ug/ml for a molecule or protein in serum or plasma .how to dilute and what is the initial volume to be taken for quantitative analysis
There are a number of things to consider here. It is very important to know which buffer is compatible with the ELISA kit you are using and you have to stick to that buffer. Sometimes, this buffer comes with the kit or could be suggested by the kit maker.
Having said that, I will recommend you do a serial dilution of your samples. You could first do a 10-fold dilution. Then do a second 10-fold dilution of the diluted sample. And lastly, another 10-fold dilution to give a total of 1000-fold dilution. With this, you should be able to detect your target protein/molecule within the ng/ml detection range.
For the volume required for quantitative analysis, if it is a 96-well ELISA plate, I would recommend at least 50 ul of your diluted sample (I assume your target protein/molecule is abundant in the sample since you could detect it in the ug/ml range). Otherwise, a 100 ul volume of the diluted sample could be just perfect.