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The Test Sample
What is being tested?
Chromosome analysis or karyotyping is a test that evaluates the number and structure of a person's chromosomes in order to detect abnormalities. Chromosomes are thread-like structures within each cell nucleus and contain the body's genetic blueprint. Each chromosome contains thousands of genes in specific locations. These genes are responsible for a person’s inherited physical characteristics and they have a profound impact on growth, development, and function.
Humans have 46 chromosomes, present as 23 pairs. Twenty-two pairs are found in both sexes (autosomes) and one pair (sex chromosomes) is present as either XY (in males) or XX (in females). Normally, all cells in the body that have a nucleus will contain a complete set of the same 46 chromosomes, except for the reproductive cells (eggs and sperm), which contain a half set of 23. This half set is the genetic contribution that will be passed on to a child. At conception, half sets from each parent combine to form a new set of 46 chromosomes in the developing fetus.
Chromosomal abnormalities include both numerical and structural changes. For numerical changes, anything other than a complete set of 46 chromosomes represents a change in the amount of genetic material present and can cause health and development problems. For structural changes, the significance of the problems and their severity depends upon the chromosome that is altered. The type and degree of the problem may vary from person to person, even when the same chromosome abnormality is present.
A chromosomal karyotyping examines a person's chromosomes to determine if the right number is present and to determine if each chromosome appears normal. It requires experience and expertise to perform properly and to interpret the results. While theoretically almost any cells could be used to perform testing, in practice it is usually performed on amniotic fluid to evaluate a fetus and on lymphocytes (a white blood cell) from a blood sample to test all other ages. Alternately, white blood cells may be obtained from bone marrow aspirations to look for changes in individuals suspected of having hematologic or lymphoid diseases (e.g., leukemia, lymphoma, myeloma, refractory anemia).
The test is performed by:
- Taking a sample of a person's cells, culturing them in nutrient-enriched media to promote cell division in vitro. This is done in order to select a specific time during the cells' growth phase when the chromosomes are easiest to distinguish.
- Isolating the chromosomes from the nucleus of the cells, placing them on a slide, and treating them with a special stain.
- Taking microphotographs of the chromosomes.
- In jigsaw puzzle fashion, rearranging the pictures of the chromosomes to match up pairs and arrange them by size, from largest to smallest, numbers 1 to 22, followed by the sex chromosomes as the 23rd pair.
- The pictures also allow the chromosomes to be vertically oriented. Each chromosome looks like a striped straw. It has two arms that differ in length (a short arm (p) and a long arm (q)), a pinched-in area between the arms called a centromere, and a series of light and dark horizontal bands. The length of the arms and the location of the bands help determine top from bottom.
- Once the chromosome photo arrangement is completed, a laboratory specialist evaluates the chromosome pairs and identifies any abnormalities that may be present.
Some chromosomal disorders that may be detected include:
- Down syndrome (Trisomy 21), caused by an extra chromosome 21; this may occur in all or most cells of the body.
- Edwards syndrome (Trisomy 18), a condition associated with severe mental retardation; caused by an extra chromosome 18.
- Patau syndrome (Trisomy 13), caused by an extra chromosome 13.
- Klinefelter syndrome, the most common sex chromosome abnormality in males; caused by an extra X chromosome.
- Turner syndrome, caused by missing one X chromosome in females.
- Chronic myelogenous leukemia, a classic 9;22 translocation that is diagnostic of the disease.
How is the sample collected for testing?
- A blood sample is obtained by inserting a needle into a vein in the arm.
- Amniotic fluid and chorionic villi are collected from a pregnant woman by a healthcare practitioner using amniocentesis or chorionic villus sampling procedures.
- Bone marrow or tissue sample collections require a biopsy procedure to be performed.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.