Some antibodies react to epitopes shared between species. These antibodies can be used to detect the antigen of multiple species.  Other antibodies are species specific and can only detect the antigen in one species.

So do not buy a kit unless it explicitly states that it can detect rat insulin.  Some kits do not detect rat insulin.

The kit protocol should indicate the cross reaction the insulin of other species. Also, the kit should indicate cross reaction with pro-insulin and c-peptide.

The homology between the mature insulin molecule from rat and human is is high. It is uncommon to find an antibody which will exclusively bind only rat and not human. Commonly available kits (in US) seem to have this limitation, it is possible that some regional suppliers in your area may have antibodies that are more selective. Suffice it to say that you have to check the cross-reactivity profile before making a decision, especially if your samples are expected to contain both versions of insulin. For example, insulin reagents designed for rat show >160% reactivity with human insulin.

There are some kits for human version that detect rat insulin minimally. are minimally.But there are others that cross-react heavily. In the latter case, you could use it, provided your standards are derived from rat.

If distinction in the two species is necessary, you should take the advantage of greater differences between the sequence of C-peptide and choose a rat specific C-peptide kit.

It seems base on high cross reactivity between insulin rat and human and in addition regarding to small peptide insulin (51 residues) that remain in secondary structure. i suggest you can use the human kit for insulin in plasma or serum of rats.