Companion Diagnostics: integration of Diagnostic Testing into Treatment Pathway


What is Companion Diagnostic (CDx)?

Companion Diagnostic
A companion diagnostic refers to a test used for matching specific therapy. For example, a companion diagnostic test can determine whether a patient’s tumor has a specific biomarker targeted by the drug, to determine whether a specific therapy is effective on the patient. The development of new targeted cancer therapies usually come with its companion diagnostics to increase the treatment efficacy.

What is the Difference Between Companion Diagnostic and Complementary Diagnostics?

With the accumulated understanding of disease mechanisms and molecular diagnosis, the development of biomarkers has been accelerated. More and more companion diagnostic and complementary diagnostics have been approved by the U.S. Food and Drug Administration (FDA). According to the FDA’s definition, a companion diagnostic provides essential information for the safe and effective use of a corresponding project, while a complementary diagnostic provides additional information or guidance for a particular product. Patients can obtain an assessment of the risks or benefits of drugs or treatments from complementary diagnostics, but the recommendations provided by complementary diagnostics are not a prerequisite for receiving drugs or treatments.

Example of Companion Diagnostics - HER2 and EGFR

The treatment of HER2-positive breast cancer is one of the most used companion diagnostics. About 20% of breast cancer cases in Hong Kong are HER2-positive, and HER2-positive breast cancer tumors grow and spread faster than other non-HER2-positive breast cancer. Herceptin, targeted therapy for breast cancer, can only be used effectively if the HER2 gene is found to be overexpressed. With HER2 diagnostics, the first companion diagnostic in history, was approved by the FDA in 1998, has brought us to the era of companion therapy.

EGFR mutation test is another common companion diagnostic approved by the FDA in recent years for treating patients with EGFR mutations of non-small cell lung cancer (NSCLC). EGFR mutations are very common among lung cancer patients in Hong Kong. About 47% of non-small cell lung cancer patients reported an incidence of EGFR mutation-positive2. If a patient is tested to be EGFR mutation-positive, EGFR targeted therapies can be used.

Reference materials:

  1. U.S. Food & Drug Administration. Companion Diagnostics
  2. You JHS, Cho WCS, Ming WK, Li YC, Kwan CK, Au KH, Au JS. EGFR mutation-guided use of afatinib, erlotinib and gefitinib for advanced non-small-cell lung cancer in Hong Kong - A cost-effectiveness analysis. PLoS One. 2021 Mar 1;16(3):e0247860. doi: 10.1371/journal.pone.0247860. PMID: 33647045; PMCID: PMC7920377.