Chromosome Analysis (Karyotyping)
When pregnancy screening tests are abnormal; whenever signs of a chromosomal abnormality-associated disorder are present; as indicated to detect chromosomal abnormalities in a person and/or detect a specific abnormality in family members; sometimes when a person has leukemia, lymphoma, myeloma, myelodysplasia or another cancer and an acquired chromosome abnormality is suspected
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.
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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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
A chromosomal karyotype is used to detect chromosome abnormalities and thus used to diagnose genetic diseases, some birth defects, and certain disorders of the blood or lymphatic system.
It may be performed for:
- A fetus, using amniotic fluid or chorionic villi (tissue from the placenta):
- If one or more of a woman's pregnancy screening tests, such as the first trimester Down syndrome screen or the second trimester maternal serum screening, are abnormal.
- If a pregnant woman is having amniotic fluid analysis performed because she is considered at higher than normal risk of having a baby with a birth defect.
- If fetal structural and/or developmental abnormalities are detected, such as during an ultrasound.
- If there is a known chromosomal abnormality in the family line.
- A woman or a couple, prior to pregnancy, to evaluate her or their chromosomes, especially if a woman has experienced previous miscarriages or infertility.
- Tissue from a miscarriage or stillbirth, to help determine if the cause was due to a chromosomal abnormality in the fetus.
- An infant who is born with congenital abnormalities, including physical birth defects, mental retardation, delayed growth and development, or signs of a specific genetic disorder.
- A person with infertility or one who shows signs of a genetic disorder.
- Family members, to detect specific chromosomal abnormalities when they have been detected in a child or another family member.
- A person who has been diagnosed with certain types of leukemia, lymphoma, refractory anemia, or cancer as these conditions can lead to acquired changes in chromosomes; this testing may be performed on blood or a bone marrow sample.
- A fetus, using amniotic fluid or chorionic villi (tissue from the placenta):
When is it ordered?
A chromosome analysis may be ordered when a fetus is suspected of having a chromosomal abnormality, when an infant has congenital abnormalities, when a woman experiences miscarriages or infertility, and when an adult shows signs of a genetic disorder.
It may also be ordered to detect the presence of a chromosomal abnormality in family members when it has been detected in a child or in another family member.
What does the test result mean?
Interpretation of test results must be done by a person with specialized training in cytogenetics. Some findings are relatively straightforward, such as an extra chromosome 21 (Trisomy 21) indicating Down syndrome, but others may be very complex.
Although there will be typical signs with specific chromosomal abnormalities, the effects and the severity may vary from person to person and often cannot be reliably predicted.
Some examples of abnormalities that chromosome analysis may reveal include:
Trisomy This is the presence of an extra chromosome, a third instead of a pair. Diseases associated with trisomies include Down syndrome (associated with a Trisomy of chromosome 21), Patau syndrome (Trisomy 13), Edward syndrome (Trisomy 18), and Klinefelter syndrome (a male with an extra X chromosome – XXY instead of XY). Monosomy This is the absence of one of the chromosomes. An example of monosomy is Turner syndrome (a female with a single X chromosome – X instead of XX). Most other monosomies are not compatible with life. Deletions These are missing pieces of chromosomes and/or genetic material. Some may be small and difficult to be detected. Duplications These represent extra genetic material and may be present on any chromosome, such as the presence of two horizontal bands at a specific location instead of one. Translocations With translocations, pieces of chromosomes break off and reattach to another chromosome. If it is a one-to-one switch and all of the genetic material is present (but in the wrong place), it is said to be a balanced translocation. If it is not, then it is called an unbalanced translocation. Genetic Rearrangement With this, genetic material is present on a chromosome but not in its usual location. A person could have both rearrangement and duplication or deletion. An almost infinite number of rearrangements are possible. Interpreting the affects of the changes can be challenging.
Duplications, deletions, translocations, and genetic rearrangements can cause a myriad of health and development issues. It depends upon what genes are missing or are present in too many copies.
Some genetic rearrangements will be variations that do not cause noticeable symptoms. Balanced translocations (where two chromosomes have swapped portions of themselves but all of the genetic material is present) may cause no problems for the person who has them but may cause problems in their children.
Many hematologic and lymphoid malignancies (e.g., leukemia, lymphoma, myeloma, myelodysplasia) are associated with chromosomal abnormalities, which can help diagnose the disease and/or predict the clinical course of the disease.
Is there anything else I should know?
Since the sex chromosomes (XX or XY) are identified during the chromosome analysis, this test will also, as a byproduct, definitely determine the sex of a fetus.
Some chromosome alterations are too small or subtle to detect with karyotyping. Other testing technique such as fluorescent in situ hybridization (FISH) or a microarray may sometimes be performed to further investigate chromosomal abnormalities.
It is possible for people to have cells in their body with differing genetic material. This happens because of changes early in the development of a fetus that lead to the development of distinctly different cell lines and is called mosaicism. An example of this is some cases of Down syndrome. The affected person can have some cells with an extra third chromosome 21 and some cells with the normal pair.
Should everyone have this testing done?
Can a chromosome analysis be performed in my healthcare provider's office?
Why does the karyotype take several days to perform?
The cells that are tested must be cultured and cell division promoted. The amount of time that this takes will vary from sample to sample. Highly complex, abnormal karyotypes may require a longer time to evaluate.