Predicting HER2 amplification in breast cancers with equivocal HER2 immunohistochemistry.

Although reflex FISH testing is recommended for all immunohistochemistry 2+ tumors, only a subset ultimately demonstrate HER2 amplification.

Overall, 16.6% of immunohistochemistry 2+ cases were HER2 FISH positive.

A Ki-67 cutoff of 20% achieved 96% sensitivity, 97% negative predictive value, and a 4% false-negative rate.

Tumors with Ki-67 < 20% were almost uniformly FISH negative, while those with higher Ki-67 values demonstrated a progressively increased probability of HER2 amplification.