Why might testing in blood miss somatic mosaicism?

• A. Mosaicism may be confined to tissues other than blood.
• B. Blood tests are always comprehensive.
• C. Mosaicism cannot be detected by sequencing depth.
• D. Blood tests only assess cardiovascular genes.

Answer: (A)

The key idea is that somatic mosaicism can be restricted to tissues other than blood. Mosaicism arises from a mutation after fertilization, so the mutated cells may populate some tissues but not others. If the mutation is confined to tissues such as skin, brain, or others, a blood sample may show no variant, and sequencing of blood would miss the mosaic change. Blood is commonly tested because it’s easy to obtain, but its cell population doesn’t always reflect the mutational pattern across the body. The depth of sequencing can help detect low-level mosaicism in blood when the mutation is present there, but that doesn’t negate the possibility that the mutation isn’t in the blood at all. The other statements are inaccurate: sequencing depth can reveal low-level mosaicism, and blood testing isn’t limited to cardiovascular genes.