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 having blood drawn (venipuncture), 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 any child or baby who has unusual features or developmental delays that suggest a genetic abnormality.
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.
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
Chad Haldeman-Englert, MD, Wake Forest University School of Medicine, Department of Pediatrics, Section on Medical Genetics, Winston-Salem, NC. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.