Why a normal result is not a negative: copy number, pseudogenes and residual risk

“No pathogenic variant found” is one of the most reassuring sentences in a genetic report, and one of the least precise. It is a statement about the variants that were looked for, in the regions that could be analysed, by the method that was used. Everything outside that boundary is not negative; it is unexamined or only partly examined.

Three ways a normal result is narrower than it looks

Copy number. A sequence-level pipeline reports single-nucleotide changes and small indels. A whole-exon deletion, a duplicated gene or a gene missing altogether may leave nothing to call. Detecting these needs their own methods, ideally with more than one line of evidence, and a way to score the clinical relevance of what is found. Without that layer, the absence of a finding says nothing about dosage.

Pseudogenes. Where a gene has a near-identical pseudogene, reads cannot always be assigned to the right copy. In some genes and exons this can be resolved with dedicated analysis; in others it cannot with short reads. Either way, the report should say which one applies, so that a resolved locus and an unresolvable one do not look the same.

Residual risk. Even a perfectly executed, fully covered analysis has a limit. In carrier screening, a negative result lowers the probability of being a carrier; it does not bring it to zero. The same holds for a normal copy number of a gene where some carriers cannot be identified by counting copies. A report that states the residual risk gives the person reading it something they can act on.

What a report should contain

For every analysis, three things: what was evaluated, what was filtered out and why, and what was not looked at. The first tells the reader what the result covers. The second makes automated filtering, such as common population events or low-mappability regions, auditable rather than invisible. The third turns silence into information: a locus that short reads cannot resolve is declared, not guessed.

The geneticist decides, with the boundary in view

None of this replaces professional judgement; it feeds it. With the boundary visible, the reviewing geneticist can accept a result, order an orthogonal assay for a region that could not be resolved, or write the limitation into the signed report. Without it, the same decision is made blind.