Trisomy 18, mosaicism (mitotic nondisjunction)

This code represents a genetic disorder characterized by the presence of an extra copy of chromosome 18 in some, but not all, cells of the body. This mosaicism results from a mitotic nondisjunction event, meaning the error occurred after fertilization during early cell division. The clinical presentation of mosaic Trisomy 18 can vary widely, often being less severe than full Trisomy 18.

Use this code when documentation specifically identifies a diagnosis of Trisomy 18 with mosaicism. This diagnosis is typically confirmed through cytogenetic testing, such as karyotyping or FISH, showing a mixed cell population with both normal and trisomic cells. It is appropriate for both prenatal and postnatal diagnoses.

• Differentiate carefully between full Trisomy 18 (Q91.3) and mosaic Trisomy 18.
• Ensure the medical record explicitly states "mosaicism" or "mitotic nondisjunction" to support this specific code.
• Do not use this code for partial Trisomy 18 (Q91.2), which involves only a segment of chromosome 18.