Karyotyping is a test to examine chromosomes in a sample of cells, which can help identify genetic problems as the cause of a disorder or disease. This test can:
- Count the number of chromosomes
- Look for structural changes in chromosomes
How the test is performed
The test can be performed on almost any tissue, including:
To test amniotic fluid, an amniocentesis is done.
A bone marrow biopsy is needed to take a sample of bone marrow.
The sample is placed into a special dish or tube and allowed to grow in the laboratory. Cells are later taken from the new sample and stained. The laboratory specialist uses a microscope to examine the size, shape, and number of chromosomes in the cell sample. The stained sample is photographed to shows the arrangement of the chromosomes. This is called a karotype.
Certain problems can be identified through the number or arrangement of the chromosomes. Chromosomes contain thousands of genes that are stored in DNA, the basic genetic material.
How to prepare for the test
There is no special preparation needed.
How the test will feel
How the test will feel depends on whether the sample procedure is having blood drawn (venipuncture), amniocentesis, or bone marrow biopsy.
Why the test is performed
This test may be done:
- On a couple that has a history of miscarriage
- To examine any child or baby who has unusual features or developmental delays
The bone marrow or blood test can be done to identify the Philadelphia chromosome, which is found in about 85% of people with chronic myelogenous leukemia (CML).
The amniotic fluid test is done to check a developing baby for chromosome problems.
- Females: 44 autosomes and 2 sex chromosomes (XX), written as 46, XX
- Males: 44 autosomes and 2 sex chromosomes (XY), written as 46, XY
What abnormal results mean
Abnormal results may be due to a genetic syndrome or condition, such as:
This list is not all-inclusive.
Additional conditions under which the test may be performed:
What the risks are
The risks are related to the procedure used to obtain the sample. Procedures that may be done to take the sample include:
In some cases, a problem may occur as the cells as growing in the lab dish. Karyotype tests should be repeated to confirm that an abnormal chromosome problem is actually in the body of the patient.
Chemotherapy may cause chromosome breaks that affect normal karotyping results.
Your doctor may also order other tests that go together with a karyotype:
- Microarray -- looks at small changes in the chromosomes
- Fluorescent in situ hybridisation (FISH) -- looks for small mistakes such as deletions in the chromosomes