The Purpose and Steps Involved in a Karyotype Test
If your health care provider has recommended a karyotype test for you or your child, or after amniocentesis, what does this test entail? What conditions can a karyotype diagnose, what steps are involved in performing the tests, and what are its limitations?
What is a karyotype test?
A karyotype is a photograph of the chromosomes of a cell. Karyotypes can be taken from blood cells, fetal skin cells (from amniotic fluid or placenta), or bone marrow cells.
Conditions diagnosed with a karyotype test
Karyotypes can be used to screen for and confirm chromosomal abnormalities such as Down syndrome and cat’s eye syndrome, and several types of abnormalities can be detected.
- Trisomies in which there are three copies of one of the chromosomes instead of two
- Monosomies in which only one copy (instead of two) is present
- Chromosomal deletions in which part of a chromosome is missing
- Chromosomal translocations in which part of a chromosome is attached to another chromosome (and vice versa in balanced translocations.)
Here are some examples of trisomies:
An example of monosomy includes:
- Turner syndrome (X0) or monosomy X – About 10% of first trimester miscarriages are due to Turner syndrome, but this monosomy is only present in about 1 in 2,500 live births.
Examples of chromosomal deletions include:
Translocations – There are many examples of translocations, including the Down syndrome translocation. Robertsonian translocations are quite common and occur in about 1 in 1000 people.
Mosaicism is a condition in which some cells in the body have a chromosomal abnormality while others do not. For example, mosaic Down syndrome or mosaic trisomy 9. Complete trisomy 9 is not compatible with life, but mosaic trisomy 9 can result in a live birth.
When it’s done
There are many situations in which a karyotype may be recommended by your healthcare provider. These could include:
- Infants or children who have medical conditions suggesting a chromosomal abnormality that has not yet been diagnosed.
- Adults who have symptoms suggestive of a chromosomal abnormality (for example, males with Klinefelter’s disease may not be diagnosed until puberty or adulthood.) Some of the mosaic trisomy disorders may also not be diagnosed.
- Infertility: A genetic karyotype can be done for infertility. As noted above, some chromosomal abnormalities may go undiagnosed until adulthood. A woman with Turner syndrome or a man with one of the variants of Klinefelter disease may not be aware of the condition until they face infertility.
- Prenatal testing: In some cases, such as translocation Down syndrome, the condition may be inherited and parents may be tested if a child was born with Down syndrome. (It is important to note that most of the time Down syndrome is not an inherited disorder but rather a chance mutation.)
- Stillbirth: A karyotype is often done as part of testing after a stillbirth.
- Recurrent miscarriages: A parental karyotype of recurrent miscarriages can give clues to the reasons for these devastating recurrent losses. It is believed that chromosomal abnormalities, such as trisomy 16, are the cause of at least 50% of miscarriages.
- Leukemia: Karyotype tests may also be done to help diagnose leukemias, such as looking for the Philadelphia chromosome found in some people with chronic myeloid leukemia or acute lymphocytic leukemia.
A karyotype test can look like a simple blood test, which makes many people wonder why it takes so long to get the results. This test is actually quite complex after collection. Let’s take a look at these steps so you can understand what’s going on while you wait for the test.
The first step in performing a karyotype is to take a sample. In newborns, a blood sample containing red blood cells, white blood cells, serum, and other fluids is taken. A karyotype will be performed on white blood cells that are actively dividing (a condition known as mitosis). During pregnancy, the sample can be either amniotic fluid taken during amniocentesis or a piece of placenta taken during a chorionic villus sampling (CVS) test. Amniotic fluid contains fetal skin cells which are used to generate a karyotype.
2. Transport to the laboratory
Karyotypes are done in a specific lab called a cytogenetics lab, a lab that studies chromosomes. Not all hospitals have cytogenetics laboratories. If your hospital or medical facility does not have its own cytogenetics laboratory, the test sample will be sent to a laboratory that specializes in karyotype analysis. The test sample is analyzed by specially trained cytogenetic technologists, Ph.D. cytogeneticists or medical geneticists.
3. Separate cells
In order to analyze chromosomes, the sample must contain actively dividing cells. In the blood, white blood cells are actively dividing. Most fetal cells are also actively dividing. Once the sample reaches the cytogenetics laboratory, cells that are not dividing are separated from dividing cells using special chemicals.
4. Growing cells
In order to have enough cells to analyze, dividing cells are grown in special media or cell culture. This medium contains chemicals and hormones that allow cells to divide and multiply. This culture process can take three to four days for blood cells and up to a week for fetal cells.
5. Cell synchronization
Chromosomes are a long string of human DNA. To see chromosomes under a microscope, the chromosomes must be in their most compact form in a phase of cell division (mitosis) called metaphase. In order to bring all cells to this specific stage of cell division, the cells are treated with a chemical that stops cell division at the point where the chromosomes are most compact.
6. Release chromosomes from their cells
To see these compact chromosomes under the microscope, the chromosomes must be outside the white blood cells. This is done by treating the white blood cells with a special solution that causes them to burst. This is done while the cells are on a microscopic slide. The remaining white blood cell debris is washed away, leaving the chromosomes stuck to the slide.
7. Chromosome Staining
Chromosomes are naturally colorless. In order to distinguish one chromosome from another, a special stain called Giemsa stain is applied to the slide. Giemsa stain stains regions of chromosomes rich in adenine (A) and thymine (T) bases. When stained, the chromosomes look like strings with light and dark bands. Each chromosome has a specific pattern of light and dark bands that allow the cytogeneticist to distinguish one chromosome from another. Each dark or light band encompasses hundreds of different genes.
Once the chromosomes are stained, the slide is placed under the microscope for analysis. An image is then taken of the chromosomes. At the end of the analysis, the total number of chromosomes will be determined and the chromosomes sorted by size.
9. Count the chromosomes
The first step in the analysis is to count the chromosomes. Most humans have 46 chromosomes. People with Down syndrome have 47 chromosomes. It is also possible for people to have missing chromosomes, more than one extra chromosome, or part of a missing or duplicated chromosome. By looking only at the number of chromosomes, it is possible to diagnose different conditions, including Down syndrome.
10. Chromosome sorting
After determining the number of chromosomes, the cytogeneticist will begin sorting the chromosomes. To sort the chromosomes, a cytogeneticist will compare the length of the chromosomes, the placement of the centromeres (the areas where the two chromatids are joined), and the location and size of the G bands. Chromosome pairs are numbered from the largest (number 1 ) to the smallest (number 22). There are 22 pairs of chromosomes, called autosomes, which match exactly. There are also sex chromosomes, females have two X chromosomes while males have one X and one Y.
11. Look at the structure
In addition to looking at the total number of chromosomes and sex chromosomes, the cytogeneticist will also look at the structure of specific chromosomes to ensure there is no missing or extra material as well as structural abnormalities such as translocations . A translocation occurs when part of a chromosome is attached to another chromosome. In some cases, two pieces of chromosomes are swapped (a balanced translocation) and other times an extra piece is added or missing from a single chromosome.
12. The End Result
Ultimately, the final karyotype shows the total number of chromosomes, sex, and any structural abnormalities with individual chromosomes. A digital image of the chromosomes is generated with all the chromosomes ordered by number.
Limitations of the karyotype test
It is important to note that although the karyotype test can give a lot of information about chromosomes, this test cannot tell you if specific genetic mutations, such as those that cause cystic fibrosis, are present. Your genetic counselor can help you understand both what karyotype tests can and cannot tell you. Further studies are needed to assess the possible role of genetic mutations in the disease or miscarriages.
It is also important to note that sometimes karyotype tests may not be able to detect certain chromosomal abnormalities, such as the presence of placental mosaicism.
Currently, karyotype testing in the prenatal setting is quite invasive, requiring amniocentesis or chorionic villus sampling. However, the evaluation of cell-free DNA in a mother’s blood sample is now common as a much less invasive alternative for prenatal diagnosis of genetic abnormalities in a fetus.
A word from Verywell
While waiting for your karyotype results, you may feel very anxious and the week or two it takes to get results can seem like a long time. Take this time to lean on your friends and family. Learning about some of the conditions associated with abnormal chromosomes can also be helpful. Although many conditions diagnosed with a karyotype can be devastating, many people living with these conditions have an excellent quality of life.