Can I use recombinant proteins from different companies for my ELISA?

You could use recombinant proteins from different companies for your ELISAs, however keep the following in mind:

1. Protein tags are useful for protein purification.
2. Recombinant proteins expressed from E.coli from the same source can show greater than 10 fold difference in terms of immunoreactivity from lot to lot, primarily due to refolding inconsistency.
3. Different kit standards can be produced and calibrated against different references. So far there is no universally accepted standardization for recombinant protein immunoreactivities.
4. Any changes to the reagents (standards, antibodies, matching matrices) and protocols may affect the final assay performance.