CareDx Inc. has been on a big run. Over the last year, the molecular diagnostics company has seen its stock price quintuple, yielding a $1.5 billion market cap. The driver has been its patented method for assessing organ transplant rejections with a simple blood test.

Now other companies are moving into the market. Last month Natera Inc. announced a partnership with Thermo Fisher Scientific to market its own diagnostic test for kidney transplants.