How is Mosaicism Diagnosed?
The usual way in which mosaic Down syndrome is discovered is through genetic testing of the baby’s blood. Typically, 20 to 25 cells are examined. If some of the cells have trisomy 21 and some don’t, then the diagnosis of mosaicism is made. However, this blood test can only determine the level of mosaicism in the blood cell line. While mosaicism can occur in just one cell line (some blood cells have trisomy 21 and the rest don’t), it can also occur across cell lines (skin cells may have trisomy 21 while other cell lines don’t). In the latter case, it may be more difficult to diagnose mosaicism. When mosaicism is suspected but not confirmed through the blood test, other cell types may be tested: skin and bone marrow are most commonly the next cells checked. Because skin cells and brain cells arise from the same type of cell at the beginning of fetal development (ectoderm), many doctors believe that skin cell tests reflect the chromosomal makeup of the brain cells as well.