InSight: Fluorescence in situ Hybridization (FISH), Prenatal Aneuploid Evaluation, Amniotic Fluid With Reflex to Microarray or Chromosome Analysis

Amniotic fluid

25 mL or greater

25 mL

Sterile plastic conical tube

The patient preferably should have had ultrasound studies (to verify fetal life, detect multiple gestation, confirm gestational age, localize fetus/placenta).

Rapid identification of common prenatal aneuploidy (specific for X, Y, 13, 18, 21). If abnormal, reflex to banded chromosomes to obtain fetal karyotype. This test allows prenatal detection of chromosomal rearrangements, aneuploidy, or mosaicism. Such groups include women who:

• are age 35 years or older

• have a previous child having chromosome abnormality or multiple congenital abnormalities

• have had two or more previous spontaneous abortions

• have a family history of a chromosome abnormality

• are known carriers of an X-linked disorder

• are 31 years of age or older with twin pregnancies

• have abnormal fetal ultrasound findings

• have a positive maternal serum marker screen

Additional biochemical or molecular tests may be performed on the cultured amniocytes. Fetal loss rate at 14 to 18 week sampling is considered to be 0.5%, and 2% to 3% at 10 to 13 weeks. Chorionic villus sampling (CVS) may be safer than early amniocentesis for early prenatal diagnosis of cytogenetic abnormalities. The risk of miscarriage with CVS is 1% to 1.5% but the risk of maternal infection appears to be higher with CVS than with amniocentesis. Cytogenetic analyses using such samples allow for an early gestational testing based on a 10- to 11-week placental biopsy and a 8-day cytogenetic study. Most failures are due to an inappropriate biopsy containing only maternal decidua.

Chromosomal aberration were found in 4.6% of fetuses in women older than 38 to 40 years of age. Trisomy 21 was the most common abnormality (62%). Klinefelter syndrome (11%) and trisomy 18 (11%) were next most frequent in the cases of advanced maternal age. Prenatal diagnosis is possible for more than 1000 inherited diseases. Most are inherited in an autosomal recessive manner. Antenatal molecular diagnosis has become available for cystic fibrosis, muscular dystrophy, sickle cell anemia, hemophilia, and many other genetic abnormalities. This can be done from either cultured amniotic fluid cells or chorionic villous sampling. A normal FISH result reflexes to the SNP assay, which will detect chromosomal imbalances that could be associated with developmental delay/congenital anomalies. It provides detection of possible uniparental disomy of any chromosome, the percent and location of homozygosity, including the degree of identity by descent.

FISH detects only the most common aneuploidies found in the second trimester. Abnormal results will reflex to banded chromosome analysis. Although the overall culture success rate is reported as >99%, culture failure can result. Reasons include, but are not limited to lack of amniocytes in the fluid, and contamination of the fluid with bacteria or yeast. Normal FISH results will reflex to the microarray. FISH will not detect balanced rearrangements and may not detect low level mosaicism. The SNP assay does not detect balanced rearrangements, low-level mosaicism (<10%), marker chromosomes that only contain heterochromatin or tetraploidy. Extensive maternal cell contamination will limit the sensitivity of the assay.

Fluorescence in situ hybridization (FISH ) and in situ chromosome cell culturing of amniocytes to investigate numerical and/or structural chromosome abnormalities. SNP microarray analysis from uncultured cells is performed using the Cytoscan® HD platform, which uses more than 743,000 SNP probes and 1,953,000 NPCN probes with a median spacing of 0.88 kb. If DNA yield on uncultured cells is inadequate, analysis will be performed on cultured cells. If cultures are needed, additional days will be required to complete testing.