I would like to know what comes after the ELISA ASSAY has detected a
protein in a test tube during protein purification. What is the next
step.

Why are your questions so cryptic?  

If you can do an Elisa test is is easy for you to assay for the
concentration of your protein in a complex mixture.  But what are your
long term aims?

1) Do you want to purify the protein? If you have an antibody that you
use for the Elisa Assay then someone has already worked out a
protocol.

2) Is this a diagnosis where you need to know the level of protein in
a sample?  If that is the case you don't need to do anything else.

3) Do you want to change the function of the protein?  If so, you'll
need to clone it, use site directed mutagenesis and put it back into
an organism as a transgene.

'What's next' depends on your question.  Stop being cryptic and tell
us your hypothesis.

By the way your question does not make too much sense.  Elisa tests
are not part of a purification protocol.  Elisa tests are used to
quantify a protein in a complex mixture of proteins. An Elisa assay is
something that is established after the first purification protocol
has been worked out.

It would help to know why you want to purify this protein. Since you
know what the protein is why don't you just clone it and express it in
E.coli as a recombinant protein?  At present your questions are
impossible to answer since you do not outline your rationale.