Therefore to answer the second part of this question — which presumably refers to the interferon gamma (IFNγ) gene test offered by AnimalLabs, Croatia, and Genimal Biotechnologies in France — it is my view that it would be much wiser to test cats for FCoV antibodies and (unless antibody negative) do monthly
faecal RT - PCR tests in order to
eliminate FCoV from breeding cats and so prevent cats from encountering FCoV, rather than waste money on FIP DNA tests which will give you very little practical information.