What Is a Karyotype Test and How Is it Done?

Purpose and Steps Involved in a Karyotype Test

Human karyotype, artwork
somersault18:24 / Getty Images

If your doctor has recommended a karyotype test for you or your child, or after an amniocentesis, what does this test entail? What conditions may a karyotype diagnose, what are the steps involved in doing the tests, and what are its limitations?

What Is a Karyotype Test?

A karyotype is a photograph of the chromosomes in a cell. Karyotypes can be taken from blood cells, fetal skin cells (from amniotic fluid or the placenta), or bone marrow cells.

What Conditions May Be Diagnosed With a Karyotype Test?

Karyotypes can be used to screen for and confirm chromosomal abnormalities such as Down's syndrome, and there are several different types of abnormalities which may be detected.

One of these is trisomies in which there are three copies of one of the chromosomes rather than two. In contrast, monosomies occur when only one copy (instead of two) is present. In addition to trisomies and monosomies there are chromosome deletions in which part of a chromosome is missing, and chromosome translocations, in which a part of one chromosome is attached to another chromosome (and vice versa in balanced translocations.)

Examples of trisomies include:

  • Down syndrome (trisomy 21)
  • Edward syndrome (trisomy 18)
  • Patau syndrome (trisomy 13)
  • Klinefelter's syndrome (XXY and other variations) - Klinefelter's syndrome occurs in 1 in 500 newborn males and appears to be increasing in incidence
  • Triple X syndrome (XXX)

An example of a monosomy includes:

  • Turner syndrome (X0) or monosomy X - Roughly 15 percent of miscarriages are due to Turner syndrome, but this trisomy is present in only around 1 in 2000 live births

Examples of chromosomal deletions include:

  • Cri-du-Chat syndrome (missing chromosome 5)
  • Williams syndrome (missing chromosome 7)

Translocations - There are many examples of translocations including translocation Down syndrome. Robertsonian translocations are fairly common, occurring in roughly 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. Full trisomy 9 is not compatible with life, but mosaic trisomy 9 may result in a live birth.

(An example is worth a thousand words. Learn about the differences between translocation, trisomy, and mosaic Down syndrome.)

When Is a Karyotype Done?

There are many situations in which a karyotype may be recommended by your physician. These might include:

  • Infants or children who have medical conditions which suggest a chromosomal abnormality that has not yet been diagnosed.
  • Adults who have symptoms suggestive of a chromosomal abnormality (for example, men with Klinefelter's disease may go undiagnosed until puberty or adulthood.) Some of the mosaic trisomy disorders may also go undiagnosed.
  • Infertility - A genetic karyotype may 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's may not be aware of the condition until they are coping with infertility.
  • Prenatal testing - In some cases, such as translocation Down syndrome, the condition may be hereditary and parents may be tested if a child has been born with a Down syndrome. (It's important to note that most of the time Down syndrome is not a hereditary disorder but rather a chance mutation.)
  • Stillbirth - A karyotype is often done as part of the testing following a stillbirth.
  • Recurrent miscarriages - A parental karyotype of recurrent miscarriages may give clues as to the reasons for these devastating recurring losses. It's thought that chromosomal abnormalities, such as trisomy 16 are the cause of a least 50 percent of miscarriages.
  • Leukemia - Karyotype testing may also be done to help diagnose leukemias, for example, by looking for the Philadelphia chromosome found in some people with chronic myelogenous leukemia or acute lymphocytic leukemia.

Steps Involved in a Karyotype Test

A karyotype test may sound 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 these steps so you can understand what is happening during the time you are waiting for the test.

1. Sample Collection

The first step in performing a karyotype is to collect a sample. In newborns, a blood sample containing red blood cells, white blood cells, serum, and other fluids is collected. A karyotype will be done on the white blood cells which are actively dividing (a state known as mitosis). During pregnancy, the sample can either be amniotic fluid collected during an amniocentesis or a piece of the placenta collected during a chorionic villi sampling test (CVS). The amniotic fluid contains fetal skin cells which are used to generate a karyotype.

2. Transport To the Laboratory

Karyotypes are performed in a specific laboratory called a cytogenetics lab––a lab which studies chromosomes. Not all hospitals have cytogenetics labs. If your hospital or medical facility doesn’t have its own cytogenetics laboratory, the test sample will be sent to a lab that specializes in karyotype analysis. The test sample is analyzed by specially trained cytogenetic technologists, Ph.D. cytogeneticists, or medical geneticists.

3. Separating the Cells

In order to analyze chromosomes, the sample must contain cells that are actively dividing. In blood, the white blood cells actively divide. Most fetal cells actively divide as well. Once the sample reaches the cytogenetics lab, the non-divided cells are separated from the dividing cells using special chemicals.

4. Growing Cells

In order to have enough cells to analyze, the dividing cells are grown in special media or a cell culture. This media contains chemicals and hormones that enable the cells to divide and multiply. This process of culturing can take three to four days for blood cells, and up to a week for fetal cells.

5. Synchronizing Cells

Chromosomes are a long string of human DNA. In order to see chromosomes under a microscope, chromosomes have to be in their most compact form in a phase of cell division (mitosis) known as metaphase. In order to get all the cells to this specific stage of cell division, the cells are treated with a chemical which stops cell division at the point where the chromosomes are the most compact.

6. Releasing the Chromosomes From Their Cells

In order to see these compact chromosomes under a microscope, the chromosomes have to be out of 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 leftover debris from the white blood cells is washed away, leaving the chromosomes stuck to the slide.

7. Staining the Chromosomes

Chromosomes are naturally colorless. In order to tell one chromosome from another, a special dye called Giemsa dye is applied to the slide. Giemsa dye stains regions of chromosomes that are rich in the bases adenine (A) and thymine (T). When stained, the chromosomes look like strings with light and dark bands. Each chromosome has a specific pattern of light and dark bands which enable the cytogeneticist to tell one chromosome from another. Each dark or light band encompasses hundreds of different genes.

8. Analysis

Once chromosomes are stained, the slide is put under the microscope for analysis. A picture is then taken of the chromosomes. By the end of the analysis, the total number of chromosomes will be determined and the chromosomes arranged by size.

9. Counting Chromosomes

The first step of the analysis is counting 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 a portion of a chromosome that is either missing or duplicated. By looking at just the number of chromosomes, it is possible to diagnose different conditions including Down syndrome. 

10. Sorting Chromosomes

After determining the number of chromosomes, the cytogeneticist will start sorting the chromosomes. To sort the chromosomes, a cytogeneticist will compare chromosome length, the placement of centromeres (the areas where the two chromatids are joined), and the location and sizes of G-bands. The chromosomes pairs are numbered from largest (number 1) to smallest (number 22). There are 22 pairs of chromosomes, called autosomes, which match up exactly. There are also the sex chromosomes, females have two X chromosomes while males have an X and a Y.

11. Looking at the Structure

In addition to looking at the total number of chromosomes and the sex chromosomes, the cytogeneticist will also look at the structure of the specific chromosomes to make sure that there is no missing or additional material as well as structural abnormalities like translocations. A translocation occurs when a part of one chromosome is attached to another chromosome. In some cases, two pieced of chromosomes are interchanged (a balanced translocation) and other times an extra piece is added or missing from one chromosome alone.

12. The Final Result

In the end, the final karyotype shows the total number of chromosomes, the sex, and any structural abnormalities with individual chromosomes. A digital picture of the chromosomes is generated with all of the chromosomes arranged by number.

Limits of Karyotype Testing

It's important to note that while karyotype testing can give a lot of information on chromosomes, this test cannot tell you whether specific gene mutations, such as those which cause cystic fibrosis, are present. Your genetic counselor can help you understand both what karyotype tests can tell you and what they cannot. Further studies are needed to evaluate the possible role of gene mutations in disease or miscarriages.

It's also important to note that at times karyotype testing may not be able to detect some chromosomal abnormalities, such as when placental mosaicism is present.

The Future

At the current time, karyotype testing in the prenatal setting is quite invasive, requiring amniocentesis or chorionic villus sampling. Studies are in progress evaluating cell-free DNA in a mother's blood sample as a much less invasive alternative for the prenatal diagnosis of genetic abnormalities in a fetus.

Bottom Line on Waiting for Your Karyotype Results

While waiting for your karyotype results you may be feeling very anxious, and the week or two it takes to get results can feel like eons. Take that time to lean on your friends and family. Learning about some of the conditions associated with abnormal chromosomes may also be helpful. Though many of the conditions diagnosed with a karyotype can be devastating, there are many people living with these conditions who have an excellent quality of life.

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