Companion Diagnostic (CDx)

Key facts

Regulated by

FDA Center for Devices and Radiological Health (CDRH)

Core FDA guidance

In Vitro Companion Diagnostic Devices, final August 2014

Typical pathway

Premarket approval (PMA); some via 510(k) or De Novo

Published in

FDA list of authorized companion diagnostic devices

What is a companion diagnostic?

A companion diagnostic is a test whose result is required to use a drug appropriately. It can identify patients most likely to benefit, identify patients at higher risk of serious side effects, or monitor response so treatment can be adjusted.

The drug label and the test labeling refer to each other. When a label says a therapy is indicated for patients whose tumors have a specific variant "as detected by an FDA-approved test," the test that fills that role is the companion diagnostic.

How companion diagnostics are developed and approved

The FDA encourages drug and test sponsors to develop the two products together so the test is available when the drug is approved. The drug is reviewed by the Center for Drug Evaluation and Research (CDER) or the Center for Biologics Evaluation and Research (CBER), and the test by CDRH, with coordinated timing.

• Most in vitro companion diagnostics are high-risk devices that go through premarket approval (PMA).
• The FDA list of authorized devices records each test, its biomarker, sample type, linked drug and approval date.
• In oncology, FDA guidance allows some tests to be labeled for a group of therapies that share a biomarker, rather than one drug at a time.

Why companion diagnostics matter

A drug with a companion diagnostic cannot reach its eligible patients faster than testing does. Launch forecasts depend on testing rates, turnaround time, laboratory access and payer coverage for the test, including Medicare coverage of next-generation sequencing tests under National Coverage Determination 90.2.

For competitive intelligence, companion diagnostic approvals show which biomarkers a drug maker is committed to and which laboratories and platforms are positioned around a therapy class.

Companion vs complementary diagnostics

A complementary diagnostic gives information that can help weigh the benefits and risks of a drug but is not required to prescribe it. A companion diagnostic, by contrast, is essential to safe and effective use. Laboratory developed tests may measure the same biomarker, but only an FDA-authorized test is listed as the companion diagnostic.