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

The test can be performed on almost any tissue, including:

• Amniotic fluid
• Blood
• Bone marrow
• Tissue from the organ that develops during pregnancy to feed a growing baby (placenta)

To test amniotic fluid, an amniocentesis is done.

A bone marrow specimen requires a bone marrow biopsy.

The sample is placed into a special dish and allowed to grow in the laboratory. Cells are later taken from the growing 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 provide a karyotype, which shows the arrangement of the chromosomes.

Certain abnormalities 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.

There is no special preparation needed.

How the test will feel depends on whether the sample procedure is having blood drawn (venipuncture), amniocentesis, or bone marrow biopsy.

This test is usually done to evaluate a couple with a history of miscarriages, or to examine any child or baby who has unusual features or developmental delays that suggest a genetic abnormality.

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 fetus for chromosome abnormalities.

• Females: 44 autosomes and 2 sex chromosomes (XX), written as 46, XX
• Males: 44 autosomes and 2 sex chromosomes (XY), written as 46, XY

Abnormal results may be due to a genetic syndrome or condition, such as:

• Down syndrome
• Klinefelter syndrome
• Philadelphia chromosome
• Trisomy 18
• Turner syndrome

This list is not all-inclusive.

Additional conditions under which the test may be performed:

• Ambiguous genitalia
• Chronic myelogenous leukemia (CML) or other leukemias
• Developmental delays
• Multiple birth defects

The risks are related to the procedure used to obtain the specimen.

See:

• Amniocentesis
• Bone marrow biopsy
• Chorionic villus sampling
• Venipuncture

In some cases, an abnormality 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.

See also: Mosaicism

Your doctor may also order other tests that go together with a karyotype:

• Telomere studies -- look at the ends of the chromosomes
• Microarray -- looks at small changes in the chromosomes
• Fluorescent in situ hybridisation (FISH) -- looks for small mistakes such as deletions in the chromosomes