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.
Hey hey,
A friend and I are developing a test for antibacterial resistant infections using fluorescent dyeing that is 10 times faster than existing tests. We want to know what the process is for getting clinical laboratories to adopt our product as a standard test. This information would help us customize our prototype and pitches towards the relevant parties.