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ASCO–CAP HER2 Testing Guideline

How HER2 status is determined and reported in breast (and gastric) cancer, using IHC and in-situ hybridization.

The standard for how HER2 status is tested and reported to guide HER2-targeted therapy.

What it is

Developed jointly by the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP), this guideline standardises how HER2 (human epidermal growth factor receptor 2) status is tested and reported, primarily in breast cancer, with related principles applied in gastric cancer.

Why it matters

HER2 status helps guide whether a patient may benefit from HER2-targeted therapy. Consistent testing and scoring reduce the risk of false-positive or false-negative results that could affect treatment decisions.

Key points

  • Defines immunohistochemistry (IHC) scoring as 0, 1+, 2+ or 3+
  • Equivocal (2+) IHC results are reflexed to in-situ hybridization (ISH / FISH)
  • Sets expectations for fixation, controls and interpretation
  • Promotes clear, reproducible reporting for clinical decision-making

How our reporting uses it

When HER2 testing is requested, our reporting follows the current ASCO–CAP interpretation and reflex-testing principles, so results are consistent and clinically actionable.

This page summarises a widely used external standard for general educational purposes. It is not a substitute for the full guideline, for the definitive criteria, please refer to the current published version. Our diagnostic reports reference these standards as part of a commitment to internationally consistent reporting.
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