Karyotype and FISH

Who should have prenatal testing?

Prenatal tests are often performed on women 35 years
and older because the risk of having an abnormal baby
increases with the mother’s age.

However, fetal abnormalities can also occur in a
younger woman’s pregnancy. Abnormalities can often
be suspected by nuchal translucency screening or by
bloods tests on the mother.

Prenatal tests can confirm a chromosomal abnormality
in your baby; at >11 weeks by CVS (placental biopsy) or
after 15 weeks by amniotic fluid sampling .

What is Fast FISH?

The test shows Down syndrome, Edward syndrome,
Pateau syndrome and some others. These abnormalities
account for about 85% of the chromosome
abnormalities which occur with advanced maternal
age, increased nuchal translucency thickness on
ultrasound scan and abnormal maternal serum
screening.

To show these abnormalities by FISH we do not need
to culture the cells, so the test is quicker. On uncultured
cells from the sample, fluorescent tags of DNA
can be applied to show the number of specific
chromosomes in the cells. The rest of the sample is still
sent for routine chromosome analysis (karyotype) to
check for the other rarer chromosome abnormalities.

A report will be issued for the FISH results and will be
followed by standard GTG-banded chromosome
analysis (karyotype).

This is a test for the most commonly found chromosome
abnormalities.
Fast FISH can provide results within 1-2 working days after
the sample is received in the laboratory.

The Fast FISH test detects numerical abnormalities of
chromosomes #21, 18, 13, X & Y, only.

What are chromosomes?

This is where your genes are located. The genes contain
all the information your body needs to develop and
function.

Most people have 46 chromosomes. Two of these
chromosomes (the sex chromosomes) code for gender.
A female will have two X chromosomes, while a male will
have one X and one Y chromosome. The other 22
chromosomes are paired. We inherit one of each pair
from each parent.

Karyotype of Down Syndrome

What is karyotyping?

How does the karyotyping process work?

A sample of amniotic fluid or chorionic villi is collected
and send immediately to the cytogenetics laboratory.
The sample is carefully cleaned, using sterile techniques,
so that the fetal cells can be teased out.

The cells are placed in a nutrient culture medium and
are kept at body temperature so they can divide and
multiply until we have enough to do tests on.

The process takes time and care.

After 5-8 days, the cells are ready to be examined.
Their growth is stopped at a point in the division cycle
where the chromosomes are clearly visible, called
“metaphase”.

The cells are then stained so that they patterns that
identify each different chromosome can be seen.
For each sample, at least 10-15 metaphase cells are
selected and each chromosome is analysed carefully
down the microscope by a trained cytogeneticist.

With a state-of-the-art imaging computer, the chromosomes
are matched up in pairs to make sure that all the
right chromosomes are there. Each chromosome is
examined carefully to make sure that it looks normal
and doesn’t have any extra bits or any missing pieces.
A second scientist checks each case before the report
goes out.

Cost

Limitations to the FISH test

Dr Philippa Ramsay MBBS FRANZCOG DDU COGU
Dr Linda Atkins MBBS FRANZCOG DDU
Dr Joanne Ludlow MBChB FRANZCOG FRCOG DDU
Dr Karen Mizia MBBS BSc FRANZCOG DDU
Dr Indika Alahakoon MBBS FRANZCOG DDU CMFM