Definition Chromosome analysis involves looking at cells under a microscope to see if the chromosomes are normal.
Who is a candidate for the test? People who have physical features suggesting a chromosome syndrome are candidates for this test. It is also done for people who have certain types of cancer of the white blood cells. Foetuses can be tested when there is reason to think they may have a chromosome syndrome. In some cases, relatives of people with chromosome syndromes are tested to see if there is a familial chromosome abnormality.
How is the test performed? Chromosome analysis is performed on cells taken from the person to be tested. The cells can be from blood or biopsy tissue. To test a foetus, the amniotic fluid in the womb or the placenta, the tissue that attaches the foetus to the mother, can be used.
Cells from any of these sources are put into containers in a laboratory and allowed to grow and multiply. When there are enough cells to study, the chromosomes are removed. Special stains are used to colour the chromosomes and make them visible. Then, they are studied under a microscope to see if their number and structure are normal.
Pictures of the chromosomes that have been arranged by size and shape are often called "karyotypes." When a doctors talk about a karyotype, they mean the pattern of chromosomes in a person's cells.
What is involved in preparation for the test? No special preparation is needed if the cells will be obtained from blood or biopsy material. Cells from the amniotic fluid or the placenta are obtained by amniocentesis or chorionic villus sampling (CVS). These procedures can be done only at certain times during pregnancy. Sophisticated methods are needed to collect the cells without harming the foetus. Cells from bone marrow are obtained with bone marrow biopsy. The preparations will be explained by the doctor doing these procedures.
What do the test results mean? Abnormal numbers of chromosomes cause patterns of physical features, called syndromes. Abnormalities of number do not usually run in families. Abnormal size or shape of chromosomes can also cause syndromes, and may run in families. Complex rearrangements of chromosomes often are linked with certain types of cancer of the white blood cells.
Author: Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 14/12/2004 Contributors Potential conflict of interest information for reviewers available on request
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