A variant allele fraction can only be interpreted against the proportion of tumour in the sample. A 20% fraction means something different in a sample that is 80% tumour than in one that is 25% tumour, and a low-cellularity block can produce a false negative.
Estimate and record tumour cellularity on the area to be sampled, mark that area for macrodissection, and say if the sample is near the assay's minimum.
- Record the estimated tumour cellularity of the area sent for testing
- Circle the richest tumour area for macrodissection
- Flag samples below the assay's minimum cellularity before sending
- Choose the block with least necrosis and least decalcification
Why it matters
An unreported low cellularity turns a negative molecular result into an unreliable one.
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