How accurate is whole exome sequencing, and could it miss something or come back

[Anonymous]

How accurate is whole exome sequencing, and could it miss something or come back unclear?

[Genetic Counselor]

When whole exome sequencing identifies a change in a gene that is already well understood and clearly matches a person’s symptoms, the finding is highly reliable. Across large studies, whole exome sequencing identifies a clear genetic cause in roughly 2-in-10 to 3-in-10 previously undiagnosed individuals tested for a suspected genetic condition; trio testing raises this meaningfully, with recent clinical series reporting diagnostic answers in closer to 3-in-10 to 4-in-10 families overall, rising to nearly 6-in-10 among consanguineous families, where a shared genetic background makes certain inheritance patterns easier to recognize. In pregnancy, whole exome sequencing performed after an unexplained structural finding on ultrasound identifies a genetic cause in roughly 3-in-10 cases overall, though this varies considerably by the type of anomaly seen, from around 1-in-10 for isolated heart findings to over 4-in-10 for certain skeletal findings. Families should understand, even so, that more than half of those tested will not receive a definitive genetic answer on a first round of testing.

A second important limitation is the variant of uncertain significance: a genuine, confirmed genetic change for which there is not yet enough scientific evidence to say whether it causes disease or is a harmless variation. Because whole exome sequencing reads so much genetic material at once, roughly 3-in-10 of the variants a laboratory reports back can fall into this uncertain category, and a geneticist will explain clearly when a finding is of this kind rather than a confirmed cause; classifications are regularly revisited as scientific understanding grows.

Whole exome sequencing can also uncover secondary findings: genuine, medically significant findings unrelated to the original reason for testing, such as a gene linked to a treatable heart condition or a hereditary cancer risk. The American College of Medical Genetics and Genomics maintains a specific list, currently covering 84 genes where early knowledge is considered to change medical management, and families are always given the choice, discussed during pre-test counseling, of whether they wish to receive this kind of unrelated information at all.

Finally, whole exome sequencing has real technical blind spots. Because it reads only the coding exome, and because of how the capture and sequencing process works, it cannot reliably detect repeat expansion disorders, large missing or duplicated stretches of DNA, balanced chromosome rearrangements, changes deep within non-coding regions, changes in the small separate genome carried in mitochondria, or genetic changes present in only a small fraction of a person’s cells. A geneticist may recommend a complementary test, such as chromosomal microarray or a specific single-gene study, alongside or after whole exome sequencing when the clinical picture suggests one of these blind spots may be relevant.

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