Advantages and disadvantages of secondary screening with a cell-free DNA test versus a diagnostic test in women who screen positive for trisomy 21

Secondary screening with cell-free DNA test	Diagnostic test (chorionic villus sampling or genetic amniocentesis)
Noninvasive	Invasive
Screen-positive requires confirmatory diagnostic test	Definitive
Will detect trisomy 21, 18, 13, some sex chromosome aneuploidies, and some microdeletions	Conventional G-banding will detect all numerical and all large structural chromosomal abnormalities; microdeletions and microduplications are detected by microarray
Does not screen for open neural tube defects	Amniotic fluid is tested for alpha-fetoprotein to screen for open neural tube defects
Screen-negative patients do not undergo further diagnostic testing; thus, unexpected chromosomal abnormalities that can be identified by an invasive test will be missed	Unexpected chromosomal abnormalities that can be identified by an invasive test will be detected
May delay definitive diagnosis and management since a positive screen requires further testing	Direct route to a definitive diagnosis

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