Molecular assays have minimum tumour content thresholds, and a variant present in a minority of cells can fall below detection if the sample is diluted by normal tissue and inflammation.
The pathologist's contribution is choosing the best block, circling the richest tumour area on the H&E for macrodissection, and recording an estimate of tumour cell percentage in the area to be tested. Overestimation is common, especially with heavy inflammation or mucin, so count tumour nuclei against all nuclei rather than judging by area.
- Circle the tumour-rich area on the H&E slide sent with every molecular request.
- Estimate tumour content as the percentage of tumour nuclei among all nuclei, not as tumour area.
- Record the estimate in the request and in the report.
- Discuss with the clinician before sending a sample that falls below the assay's stated threshold.
Why it matters
This one step is the most common difference between a usable molecular result and a failed or misleading one.
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