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Tests

Karyotyping

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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

Chromosome analysis

How the test is performed

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 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.

Normal Values

  • 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:

  • 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

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:

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

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.

Special considerations

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

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