Overview

    Karyotyping is a laboratory procedure that allows a physician to examine a patient’s set of chromosomes. “Karyotype” also refers to the actual collection of chromosomes being examined. Examining these chromosomes through karyotyping allows your physician to determine whether there are any abnormalities or structural problems.

    Chromosomes, which are in almost every cell of your body, contain the genetic material inherited from your parents. They are composed of deoxyribonucleic acid (DNA) and determine the way that every human being develops.

    When a cell divides, it needs to pass on a complete set of genetic instructions to each new cell it forms. Normally, when a cell is not in the process of division, the chromosomes are arranged in a diffuse, unorganized way. However, during division, the chromosomes in these new cells line up in pairs. In a karyotype test, which examines dividing cells, these pairs are arranged by their size and appearance, allowing a doctor to easily determine if any chromosomes are missing or damaged.

    Why the Test Is Useful

    If you have an unusual number of chromosomes, if they are arranged incorrectly, or if they are malformed, this can be a sign of a genetic condition, such as Down or Turner syndrome.

    Karyotyping can be used to detect myriad genetic disorders. For instance, if a woman has premature ovarian failure, she may have a chromosomal defect that karyotyping can distinguish. The test is also useful for identifying the Philadelphia chromosome, the presence of which can signal chronic myelogenous leukemia.

    Babies can be karyotype tested before they are born to diagnose genetic abnormalities that indicate serious birth defects, such as Klinefelter syndrome, in which 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. You could have blood drawn, a bone marrow biopsy (taking a sample of the spongy tissue inside certain bones), amniocentesis (taking a sample of amniotic fluid from the uterus), or another procedure.

    Rarely, complications can result from these testing methods. There is a slight risk of bleeding and infection from having blood drawn or having your bone marrow biopsied, and amniocentesis carries a very minimal risk of miscarriage.

    If you are undergoing chemotherapy, your test results may be skewed. The chemotherapy process can cause breaks in your chromosomes, which will appear in the resulting images.

    How the Test Is Performed

    The first step in karyotyping is to take a sample of your cells. The sample cells can come from a number of different tissues, including bone marrow, blood, amniotic fluid, or placenta. Sampling can be done using various methods, depending on which area of your body is being tested. If, for instance, amniotic fluid needs to be tested, the doctor will use amniocentesis to collect the sample.

    After the sample has been taken, it is placed in a laboratory dish that allows the cells to grow. A lab technician will take cells from the sample and stain them so that the chromosomes they contain can be viewed under a microscope.

    These stained cells are examined under a microscope for potential abnormalities, including:

    • 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 laboratory technician can see the chromosomes’ shape, size, and number. This information is instrumental in determining whether or not any genetic abnormalities are present.

    What the Test Results Mean

    A normal test result will show 46 chromosomes, 44 of which are autosomes (which are unrelated to determining gender) and two of which are sex chromosomes. These sex chromosomes determine one’s gender: 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 is not reflected in the patient’s actual body. In order to confirm that an abnormality is present in the patient, the karyotype test can be repeated.