Why More Mutations Do Not Necessarily Mean Better Decisions
Broader panels detect more alterations. Beyond a point, additional findings add interpretive burden faster than they add decision-relevant information.
21 July 2026 · 6 min read · Written for clinicians
There is an intuition that a larger panel is a better panel: if a narrow assay might miss something, a broader one is safer. The intuition is partly right and worth examining, because it breaks down in specific ways.
What broader panels actually add
Two things, genuinely. They cover genes that a narrower panel would not, some of which are relevant in rare situations or in tissue-agnostic contexts. And they permit composite measures — tumour mutational burden, microsatellite status, mutational signatures — that require broad sampling of the genome to compute.
Neither is trivial. In rare tumours particularly, the alteration that matters may be one a targeted panel does not cover.
What they also add
Variants of uncertain significance, in quantity. The number scales with the amount of sequence examined. Most of these will never be reclassified into anything actionable.
This creates three costs.
Interpretive load. Someone has to assess each finding. In practice, assessment quality degrades as list length grows — the twentieth variant does not get the attention the second one did.
Anchoring. A finding that is present and named exerts pull, even when the evidence behind it is weak. It is psychologically harder to disregard a specific alteration in a specific gene than to disregard an abstract possibility.
Incidental germline findings. Tumour-only sequencing detects germline variants that were not the object of the test. This raises consent, disclosure and family-implication questions that the ordering clinician may not have anticipated.
The relevant question is not panel size
It is whether the assay covers the alterations that could change management in this disease, detected by a method appropriate to their type.
This reframes the decision usefully. In lung adenocarcinoma the question is whether fusions are detected reliably — a sequence-variant panel covering three hundred genes may perform worse on ALK than a smaller fusion-capable assay. In sarcoma the question is whether the defining fusion for the suspected subtype is covered. In colorectal cancer, extended RAS and mismatch repair status carry most of the decision weight, and a very broad panel adds relatively little on top.
Depth as well as breadth
Sequencing depth determines the lowest allele fraction that can be called reliably. In a specimen with low tumour purity, or where a resistant subclone is emerging, a broad shallow assay may miss what a narrow deep one would detect.
Breadth and depth trade off against each other at a given cost. Choosing breadth by default is a choice, not a neutral position.
A more useful framing
Before ordering, it is worth asking: what decision am I trying to inform? Which alterations would change it? What type of alteration are those — sequence variants, fusions, copy-number change? Does the assay detect that type, at the sensitivity this specimen allows?
An assay chosen this way may be broad or narrow. Either way it was chosen for a reason, and the resulting report can be read against the question that prompted it.
The goal was never to detect the maximum number of alterations. It was to answer a question. More findings are only better if they bear on it.