Karyotyping is a laboratory procedure that allows your doctor to examine your set of chromosomes. “Karyotype” also refers to the actual collection of chromosomes being examined. Examining chromosomes through karyotyping allows your doctor to determine whether there are any abnormalities or structural problems within the chromosomes.
Chromosomes are in almost every cell of your body. They contain the genetic material inherited from your parents. They’re composed of DNA and determine the way every human develops.
When a cell divides, it needs to pass on a complete set of genetic instructions to each new cell it forms. When a cell isn’t in the process of division, the chromosomes are arranged in a spread out, unorganized way. During division, the chromosomes in these new cells line up in pairs.
A karyotype test examines these dividing cells. The pairs of chromosomes are arranged by their size and appearance. This helps your doctor easily determine if any chromosomes are missing or damaged.
An unusual number of chromosomes, incorrectly arranged chromosomes, or malformed chromosomes can all be signs of a genetic condition. Genetic conditions vary greatly, but two examples are Down syndrome and Turner syndrome.
Karyotyping can be used to detect a variety of genetic disorders. For example, a woman who has premature ovarian failure may have a chromosomal defect that karyotyping can pinpoint. The test is also useful for identifying the Philadelphia chromosome. Having this chromosome can signal chronic myelogenous leukemia (CML).
Babies can be karyotype tested before they’re born to diagnose genetic abnormalities that indicate serious birth defects, such as Klinefelter syndrome. In Klinefelter syndrome, a boy is born with an extra X chromosome.
Preparation and Risks
The preparation required for karyotyping depends on the method your doctor will use to take a sample of your blood cells for testing. Samples can be taken in various ways, including:
- a blood draw
- a bone marrow biopsy, which involves taking a sample of the spongy tissue inside certain bones
- an amniocentesis, which involves taking a sample of amniotic fluid from the uterus
Complications can sometimes result from these testing methods, but they’re rare. There’s a slight risk of bleeding and infection from having blood drawn or having your bone marrow biopsied. Amniocentesis carries a very minimal risk of miscarriage.
Your test results may be skewed if you are undergoing chemotherapy. Chemotherapy can cause breaks in your chromosomes, which will appear in the resulting images.
The first step in karyotyping is to take a sample of your cells. The sample cells can come from a number of different tissues. This can include:
- bone marrow
- amniotic fluid
Sampling can be done using various methods, depending on which area of your body is being tested. For example, the doctor will use amniocentesis to collect the sample if amniotic fluid needs to be tested.
After the sample has been taken, it’s placed in a laboratory dish that allows the cells to grow. A lab technician will take cells from the sample and stain them. This makes it possible for your doctor to view the chromosomes under a microscope.
These stained cells are examined under a microscope for potential abnormalities. Abnormalities can include:
- extra chromosomes
- missing chromosomes
- missing portions of a chromosome
- extra portions of a chromosome
- portions that have broken off of one chromosome and reattached to another
The lab technician can see the chromosomes’ shape, size, and number. This information is important in determining if there are any genetic abnormalities.
A normal test result will show 46 chromosomes. Two of these 46 chromosomes are sex chromosomes, which determine the sex of the person being tested, and 44 of them are autosomes. The autosomes are unrelated to determining the sex of the person being tested. Females have two X chromosomes, while males have one X chromosome and one Y chromosome.
Abnormalities that appear in a test sample could be the result of any number of genetic syndromes or conditions. Sometimes, an abnormality will occur in the lab sample that’s not reflected in your body. The karyotype test may be repeated to confirm that there’s an abnormality.