The American Society of Clinical Oncology (ASCO) and College of American Pathologists (CAP) recently issued a joint updated guideline on human epidermal growth factor receptor 2 (HER2) testing in breast cancer. The document reflects an extensive medical literature review and recommendations for testing HER2 positivity, interpreting the results, and determining HER2-targeted therapies. Originally released in 2007, the guideline was updated to strengthen and clarify prior recommendations based on evidence that has surfaced in the ensuing years. It was published in both the Journal of Clinical Oncology and the Archives of Pathology and Laboratory Medicine (J Clin Oncol 2013;31:3997–4013; Arch Pathol Lab Med 2013; doi: 10.5858/arpa.2013-0953-SA).

The 2007 document included guidance for immunohistochemistry (IHC) and fluorescence in-situ hybridization (FISH); the 2013 update also provides recommendations for bright-field ISH, which relies on a regular light microscope rather than a fluorescent one, thereby reducing some sources of variability by making it easier to identify the invasive component of specimens.

Notably, the guideline refined optimal testing algorithms for positive, equivocal, negative, and indeterminate HER2 results. For example, the 2007 guideline defined HER2 positivity as either IHC 3+ or FISH with a HER2/CEP17 ratio >2.2 or average HER2 gene copy number >6 signals/nucleus for test systems without an internal control probe. The new guideline considers HER2 positivity as IHC 3+ based on complete, intense circumferential membrane staining. The document set four thresholds for ISH positivity based on whether the test was a single- or double-probe and the average HER2 copy number signals per cell.

In the case of equivocal IHC results, the authors added to the existing IHC 2+ standard requirements about circumferential membrane staining. Criteria for equivocal FISH clarified specific copy number signals/cell for both single- and dual probe assays.

"The number of patients with equivocal results used to be rather large. But evidence suggests that the quality of HER2 testing is improving and the frequency of equivocal and inaccurate results is decreasing," said M. Elizabeth Hammond, MD, co-chair of the panel that developed the new guideline. "We believe that this is at least in part due to our earlier recommendations in 2007. We hope the current guideline will solve remaining challenges in the field, and ultimately result in better outcomes for patients with breast cancer." Hammond also is a professor of pathology at the University of Utah School of Medicine in Salt Lake City.

The authors also revised optimal test validation requirements. The 2007 document called for labs to validate their HER2 tests initially by testing, in the same lab or another laboratory, 25–100 samples by an alternative validated method. The 2013 revision calls for initial test validation to adhere to validation requirements put forth in the 2010 ASCO/CAP estrogen receptor and progesterone receptor testing guideline, including, in the case of U.S. Food and Drug Administration-approved assays, 20 negative and 20 positive samples, and in the case of lab-developed tests, 40 negative and 40 positive samples. This requirement does not apply to assays previously validated in accordance with the 2007 guideline in labs that are participating routinely in HER2 external proficiency testing programs.

The authors emphasized that labs are responsible for ensuring that their test results are reliable and accurate by complying with accreditation and proficiency testing requirements for HER2 assays.