Objectives: We compared the performance of two Food and Drug Administration-approved HER2 immunohistochemistry (IHC) tests: HercepTest (Dako) and PATHWAY anti-HER2 (4B5) (Ventana).
Materials and methods: In total, 180 invasive breast carcinomas previously tested by both HercepTest and fluorescent in situ hybridization (FISH) were retested with 4B5. Three pathologists scored the HER2 IHC using the 2013 American Society of Clinical Oncology/College of American Pathologists guidelines. The HER2 IHC results were correlated with FISH.
Results: Among 135 equivocal cases by HercepTest, 100 (74.1%) were negative by 4B5. Among 45 positive HercepTest cases 9 (20%) were equivocal by 4B5. Among 135 equivocal HercepTest results, 100 (74.1%) were nonamplified, 18 (13.3%) equivocal, and 17 (12.6%) amplified by FISH. Among the 45 positive results with HercepTest, 2 (4.5%) were nonamplified and 1 (2.2%) was equivocal by FISH. All 37 positive and 3 negative by 4B5 cases were amplified by FISH. The absolute interobserver agreement was high for both tests (Fleiss kappa=0.838 for HercepTest and 0.771 for 4B5).
Conclusions: PATHWAY anti-HER2 (4B5) significantly reduced the number of equivocal results that require additional testing. Although HercepTest was positive in a small number of HER2 nonamplified cases, 4B5 failed to detect 3 cases that were interpreted as positive by FISH, all with nonclassic or low levels of amplification.