If the big pharma company that buys your idea already has commercial relationships with those clinical laboratories, it might be pretty simply to get the clinical laboratory to offer the additional test if the test as FDA approval.

A lot of the competency of big pharma is that it has relationships like this and knows how to promote a product inside the system. Big pharma has competencies in bringing medical products to market that outsides to the system don't have.

A patent or a least a provisional one is likely important before you start pitching.