Karyotyping

Definition

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

Alternative Names

How the test is performed

The test can be performed on a sample of blood, bone marrow, amniotic fluid, or tissue from the placenta, the organ that develops during pregnancy to feed a growing baby.

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.

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 venipuncture (having blood drawn), amniocentesis, or bone marrow biopsy.

Why the test is performed

This test is usually done to evaluate a couple with a history of miscarriages or to examine an abnormal appearance of the body that suggests a genetic abnormality.

The bone marrow or blood test can be done to identify the Philadelphia chromosome, which is found in about 85% of those with chronic myelogenous leukemia (CML).

The amniotic fluid test is done to check a developing fetus for chromosome abnormalities.

Normal Values

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

What abnormal results mean

Abnormal results may be due to:

• 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:

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

What the risks are

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.

Special considerations

Chemotherapy may cause chromosome breaks that affect normal karotyping results. See: Mosaicism

Your doctor may also order a test called telomeres that looks at the ends of the chromosomes. It is often ordered along with the karyotyping test.