Diagnostic tests are a core component of modern health care practice: they determine a patient’s susceptibility to developing cancer and other disorders; they diagnose biological conditions; they monitor the progress of disease; and they can assess the risk of disease recurrence. Ensuring their innovative growth is therefore an important issue in innovation policy. While legal scholarship addresses much about the relevance of patents and other forms of intellectual property protection for diagnostic methods as a general matter, far less attention has been paid to a distinct class of diagnostic tests that deserves its own innovation policy debate: companion diagnostic tests. This note seeks to draw more attention to the economic challenges facing the companion diagnostics industry. It begins by providing the necessary background to understand what companion diagnostic tests are, and why they are vital to the future of modern healthcare. It then explores the unique underlying incentive structure amongst the key industry stakeholders, revealing how the incentives of these stakeholders are misaligned in ways that impede the industry’s growth. Relying on empirical data from case studies collected in pharmacology and biotechnology business literature, this note ultimately argues that the microeconomics of the companion diagnostics industry present a compelling case for invigorated patent protection of companion diagnostic tests.