A diagnostics technology company that created a breakthrough blood test to warn doctors of imminent kidney transplant rejection announced Wednesday it is suing a competitor trying to do the same thing.

CareDx, Inc., patented a minimally invasive blood test, AlloSure, that detects kidney rejection at least two weeks after a kidney transplant, which can warn doctors if organ rejection is looming. A rival diagnostics company, Natera, Inc., created their own blood test called Prospera and backed their development with what CareDx lawyers call a “deeply flawed” and “unreliable” study meant to fool physicians and patients.