Chromosome analysis

Alternative names
Karyotyping

Definition

Karyotyping is a test to identify chromosome abnormalities as the cause of malformation or disease. This test can:

     
  • Count the number of chromosomes  
  • Look for structural changes in chromosomes

The results may indicate genetic changes linked to a disease.

How the test is performed

The test can be performed on a sample of blood, bone marrow, amniotic fluid, or placental tissue.

Chromosomes contain thousands of genes that are stored in DNA, the basic genetic material. The specimen is grown in tissue culture in the laboratory. Then, the cells are harvested, and the chromosomes are stained and viewed under the microscope. They are 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.

How to prepare for the test
There is no special preparation for the blood test. To test amniotic fluid, an amniocentesis is performed. Testing on placental tissue is done after a chorionic villous sampling or after a miscarriage. A bone marrow specimen requires a bone marrow biopsy.

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

     
  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

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
The blood test is usually performed to evaluate a couple with a history of miscarriages or to evaluate 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 that is present in 85% of those with Chronic myelogenous leukemia (CML). The amniotic fluid test is done to evaluate a developing fetus for chromosome abnormalities.

Normal Values

     
  • females: 44 autosomes and 2 sex chromosomes (XX), denoted 46 (X,X)  
  • males: 44 autosomes and 2 sex chromosomes (XY), denoted 46 (X,Y)

What abnormal results mean
Abnormal results may show Down syndrome (trisomy 21 = three copies of chromosome 21 instead of the normal two copies), Trisomy 18, Philadelphia chromosome, Klinefelter syndrome, Turner syndrome, or other abnormalities.

Additional conditions under which the test may be performed:

     
  • chronic myelogenous leukemia (CML) or other leukemias  
  • multiple birth defects  
  • a baby born with genitals that are neither completely male or female

What the risks are

The risks are related to the procedure used to obtain the specimen. There is a specialized kind of risk in that an abnormal result may have occurred during growth of the cells after they left the body. For this reason it is often prudent to repeat the karyotype test to confirm that the abnormal chromosome constitution is in the body of the patient.

There is a rare difference between the apparent sex of the patient and their chromosomes. For example, a baby may look like it has a penis and be called a boy but turn out to have the chromosomes of a girl. This can raise issues of what gender to raise the child.

Special considerations

Chemotherapy may cause chromosome breaks interfering with normal results. Mixtures of 2 different populations of cells or chromosome constititutions are sometimes observed. This is called mosaicism and is more common in chorionic villous sampling or amniocentesis. It does not necessarily mean that the baby has abnormal chromosomes.

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.

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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