SANTA CLARA, Calif. – It has never been easy to get payers to reimburse in a timely and adequate fashion for novel diagnostics, making it notoriously difficult to build a business from them. But a few high-flying diagnostics companies, such as Madison, Wis.-based Exact Sciences Corp. and Redwood City, Calif.-based Guardant Health Inc., have been blazing the trail recently on how to rapidly scale up to become valuable commercial entities from origins as a research-based startup.