A recent study of HER2 testing in a large cohort of women with breast cancer found important limitations in testing methods.

Breast cancer is categorized into subtypes based on laboratory test results. This study looked at the accuracy in classifying breast cancers as human epidermal growth factor receptor 2 positive (HER2+) or negative (HER2–). HER2+ breast cancer is more aggressive and has a poorer prognosis than HER2– breast cancer. Accurate classification is important because specific treatments for HER2+ breast cancer are very effective in improving disease-free and overall survival. 

Tumor samples from 552 women were retested using a second method. In 22 women, the tumor type had been classified incorrectly as HER2– when in fact these tumors were HER2+.

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“There has been little research evaluating the accuracy of a negative HER2 result. What is comforting is that we found that re-testing in experienced, larger labs confirmed the original, local lab results in the majority of cases,” said Peter A. Kaufman, MD, of the Norris Cotton Cancer Center at Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire.

The repercussions of incorrectly identifying a cancer subtype are considerable. “This is an enormous issue for those who were originally classified as HER2 negative but were HER2 positive on repeat testing. HER2 targeted therapies are critically important for women with HER2 positive breast cancer,” said Kaufman.

The difference in accuracy may be due to the methods for testing in smaller versus larger pathology laboratories. Two different tests can be used to determination HER2 status: immunohistochemistry (IHC) and florescence in situ hybridization (FISH). Using both tests allowed researchers to uncover errors resulting from reliance on a single test. Of the 22 samples incorrectly categorized, 18 had been processed by a local laboratory and only one testing method was used.

This study showed the limitations of using a single HER2 testing method. It identified a group of breast cancer patients who did not receive a potentially effective therapy because their correct HER2 status was not determined on initial testing. The authors concluded that it is not advisable to rely on a single test to determine HER2 status because of inherent limitations in testing methodologies.

This study was published in Cancer (2014; doi:10.1002/cncr.28710).