Many predictive markers — MMR, HER2, PD-L1 — can vary across a tumour. Where only one block will be stained, choose the highest-grade or least differentiated area, and record which block was tested. A negative result on a low-grade focus does not exclude deficiency or positivity elsewhere.
- Identify the highest-grade focus before selecting a block.
- Record the block and grade tested in the report.
- Consider a second block if the result is negative and foci differ in grade.
- Flag heterogeneity when seen.
Why it matters
A negative on the wrong block can close off a treatment or a germline referral.
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