Table 2 HER2 immunohistochemistry results, interpretation, and consequences according to CAP/ASCP/ASCO gastrointestinal carcinoma, HERACLES and MyPathway criteria

No reactivity of membranous reactivity in < 10% of tumor cells

Score 0 - Negative. No further testing. Patient not eligible for therapy

Score 0 - Negative. No further testing. Patient not eligible for therapy

Faint/barely perceptible reactivity in ≥ 10% of tumor cells

Score 1+ - Negative . No further testing. Patient not eligible for therapy

Score 1+ - Negative. No further testing. Patient not eligible for therapy

Weak to moderate complete, basolateral or lateral membranous staining in ≥ 10% but < 50% of tumor cells

|Score 2+ - Equivocal. Perform ISH testing.

Score 2+ - Negative. No further testing. Patient not eligible for therapy

Weak to moderate complete, basolateral or lateral membranous staining in ≥ 50% of tumor cells

Score 2+ - Equivocal. Perform ISH testing

Score 2+ - Equivocal. Mandatory IHQ re-test required. If ≥ 50% cellularity confirmed, perform ISH testing. Patient eligible for therapy if ISH positive

Strong complete, basolateral or lateral membranous staining in ≥ 10% but < 50% of tumor cells

Score 3+ - Positive. No further testing. Patient eligible for therapy

Score 3+ - Positive. Mandatory IHQ re-test required. If > 10% cellularity confirmed, perform ISH testing. Patient eligible for therapy if ISH positive

Strong complete, basolateral or lateral membranous staining in ≥ 50% of tumor cells

Score 3+ - Positive. No further testing. Patient eligible for therapy

Score 3+ - Positive. No further testing. Patient eligible for therapy