Companion Diagnostics Economics: How a $200 Test Unlocks a $2B Drug

Every time an oncologist prescribes Keytruda, a diagnostics company gets paid first. The patient’s tumor gets tested for PD-L1 expression. If the test comes back positive, the drug gets prescribed. If it comes back negative, the oncologist picks something else. That test costs $200 to $400. It runs millions of times a year. Someone collects that revenue before Merck sees a dime.

Everyone buys the biotech stock. Almost nobody buys the diagnostics company that has to be used before the drug can be prescribed. That’s the edge.

The problem

Biotech investors think about drugs. They read about Phase 3 trials, PDUFA dates, and peak sales estimates. But nearly every modern targeted therapy requires a companion diagnostic (CDx), a test that identifies which patients are eligible for the drug. The drug company and the diagnostics company are linked by regulation. The FDA approved the drug and the test together. You cannot prescribe the drug without running the test.

If you only analyze the drug company, you miss half the trade.

What a companion diagnostic is

A companion diagnostic is a test required before prescribing a targeted therapy. It identifies a biomarker, a protein, gene mutation, or other molecular feature, that predicts whether the patient will respond to the drug.

Common examples:

• PD-L1 testing for Keytruda and other checkpoint inhibitors. High PD-L1 expression means the patient is more likely to respond. Our checkpoint inhibitors explainer covers this mechanism in depth.
• HER2 testing for ENHERTU, Herceptin, and other HER2-directed therapies. The tumor must express HER2 for the drug to work.
• EGFR testing for Tagrisso and other EGFR inhibitor therapies in lung cancer.
• BRCA testing for PARP inhibitors like Lynparza.
• MSI-H testing for Keytruda in solid tumors with microsatellite instability.
• NTRK testing for Vitrakvi and Rozlytrek, the tissue-agnostic approvals.

Each of these tests is a revenue stream for the diagnostics company that makes it. And each one is a regulatory tollbooth: the FDA approved the drug with the test as a companion, which means the test is baked into the drug’s label.

The co-approval model: a regulatory moat

The FDA approves the drug and the CDx simultaneously. The CDx company has a regulatory moat because the drug’s label specifies that the test must be run before prescribing. A competitor cannot simply make a cheaper generic test. They would need to go through their own FDA approval process, demonstrate analytical and clinical validity, and get their test added to the drug’s label. That takes years and millions of dollars.

This is why CDx companies have pricing power. Once your test is on a blockbuster drug’s label, you have a recurring revenue stream that lasts as long as the drug is on the market. No one can displace you without FDA approval.

The razor-and-blades economics

Think of the drug as the razor and the CDx as the blade. The drug generates the big revenue, billions per year for a blockbuster like Keytruda. The CDx generates smaller revenue per test, $200 to $4,000 depending on complexity, but it gets paid for every new patient put on the drug.

The economics compound. A drug that treats 100,000 patients per year with a $300 companion diagnostic generates $30 million per year in testing revenue. That is small compared to the drug’s revenue, but it is recurring, high-margin, and low-risk. The diagnostics company does not face binary Phase 3 trial risk. It faces the much lower risk of testing volume fluctuating with drug prescriptions.

Key players

The companion diagnostics market has several distinct business models:

• Exact Sciences (EXAS): Owns Oncotype DX, a genomic test used to guide chemotherapy decisions in breast cancer and colorectal cancer. Also owns Cologuard for colorectal cancer screening. Recurring revenue from a broad testing menu.
• Guardant Health (GH): Leader in liquid biopsy, using blood tests to detect cancer mutations. Their tests are used as companion diagnostics for targeted therapies in non-small cell lung cancer and colorectal cancer. See our liquid biopsy explainer for the technology.
• Natera (NTRA): Specializes in circulating tumor DNA testing for molecular residual disease (MRD). Their Signatera test is used to detect residual cancer after surgery, guiding decisions about whether to continue treatment. Natera’s tests are increasingly tied to drug labels.
• Foundation Medicine / Roche: Foundation Medicine makes comprehensive genomic profiling tests that identify actionable mutations across hundreds of genes. Roche acquired Foundation Medicine in 2018 for $2.4 billion, building a diagnostics-to-drug pipeline.
• Illumina (ILMN): The picks-and-shovels play. Illumina makes the sequencing machines that most CDx companies use to run their tests. If you want to own the infrastructure rather than bet on individual tests, Illumina is the exposure. They sell the drills during the gold rush.

For a stock-picking approach to the diagnostics sector, read the companion diagnostics stock roundup. For a different diagnostics business model, see our guide on PET imaging agents.

Why CDx stocks are lower-beta than therapeutic biotech

CDx companies have different risk profiles than drug developers. The recurring revenue model means they do not face binary Phase 3 trial risk. Testing volume grows as drug prescriptions grow. If a drug gets approved for a new indication, the CDx company benefits automatically because the test is on the label.

The tradeoff is lower upside. A diagnostics company will not double overnight on a clinical trial readout the way a biotech can. But it also will not lose 60% on a failed trial. CDx stocks are for investors who want exposure to the biotech market without the binary risk of drug development.

This is also why CDx companies tend to trade at premium revenue multiples. The market values recurring, predictable revenue. If you want to understand how to value these companies relative to drug developers, our biotech valuation methods guide breaks down three approaches.

How to track CDx adoption

If you are investing in CDx stocks, here is how to track adoption:

1. Check drug labels. The FDA label for any targeted therapy will specify the required companion diagnostic. If the test is on the label, the CDx company has a moat.
2. Watch NCCN guidelines. The National Comprehensive Cancer Network guidelines drive oncologist prescribing behavior. If a test is recommended in NCCN guidelines, payer coverage follows.
3. Monitor payer coverage decisions. Medicare and private insurers covering a test means volume growth. Denial means volume stagnation.
4. Track testing rates in clinical trials. Drug companies report how many patients were tested for the biomarker during enrollment. Rising testing rates mean the CDx market is growing.

The connection to accelerated approval

Many companion diagnostics are tied to accelerated approval pathways. The FDA grants accelerated approval based on a surrogate endpoint, like tumor response rate, in a biomarker-selected population. The CDx identifies that population. Without the test, the drug cannot be prescribed to the patients most likely to benefit, and the accelerated approval falls apart.

This means CDx companies benefit from the trend toward biomarker-driven drug development. As more drugs are approved based on molecular subtypes, more tests are required, and more testing revenue flows to the diagnostics companies.

Common mistakes

Buying the drug company and ignoring the CDx. If a drug requires a companion diagnostic, the CDx company captures revenue every time the drug is prescribed. Sometimes the CDx company is the better trade.

Assuming all diagnostics companies are the same. Exact Sciences, Guardant, Natera, and Foundation Medicine have different business models, different test menus, and different risk profiles. Do not lump them together.

Underestimating the regulatory moat. Once a CDx is on a drug’s label, displacing it requires a new FDA approval. That moat is deeper than most investors realize.

Final checklist

• Does the drug require a companion diagnostic before prescribing?
• Which company makes the CDx, and is it publicly traded?
• Is the CDx on the FDA label, or is it a lab-developed test?
• What is the per-test reimbursement, and how many tests run per year?
• Is the test in NCCN guidelines and covered by major payers?
• Does the drug have accelerated approval tied to the biomarker?

Companion diagnostics are the tollbooth on the highway of modern oncology. The drug gets the headlines. The test gets paid first. If you are investing in targeted therapies, you need to know who makes the test.