| Subject | Contents |
| Definition | The process of performing a karyotype which is a test to identify chromosome abnormalities as the cause of malformation or disease. This test can both count the number of chromosomes and look for structural changes in chromosomes that may indicate genetic changes associated with increased risk for disease. |
| Alternative Names | Chromosome analysis |
| How the test is performed | The test can be performed on a sample of blood, bone marrow, amniotic fluid, or placental tissue. Chromosomes contain thousands of genes that are stored in DNA, the basic genetic material. The specimen is grown in tissue culture in the laboratory; and then the cells are harvested, chromosomes are stained, and viewed under the microscope. They are photographed to provide a karyotype which shows the arrangement of the chromosomes. Abnormalities can be identified through the number or arrangement of the chromosomes. |
| How to prepare for the test | There is no special preparation for the blood test. To test amniotic fluid, an amniocentesis is performed. Testing on placental tissue is done after a chorionic villous sampling or after a miscarriage . A bone marrow specimen requires a bone marrow biopsy . Infants and children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:infant test or procedure preparation (birth to 1 year) toddler test or procedure preparation (1 to 3 years) preschooler test or procedure preparation (3 to 6 years) schoolage test or procedure preparation (6 to 12 years) adolescent test or procedure preparation (12 to 18 years) |
| How the test will feel | How the test will feel depends on the sample procedure, that is, a venipuncture , amniocentesis , or bone marrow biopsy . |
| Why the test is performed | The blood test is usually performed to evaluate a couple with a history of miscarriages or to evaluate an abnormal appearance of the body that suggests a genetic abnormality. The bone marrow or blood test can be done to identify the Philadelphia chromosome that is present in 85% of those with Chronic myelogenous leukemia (CML) . The amniotic fluid test is done to evaluate a developing fetus for chromosome abnormalities. |
| Normal Values | females 44 autosomes and 2 sex chromosomes (XX) denoted 46 (X,X) males 44 autosomes and 2 sex chromosomes (XY) denoted 46 (X,Y) |
| What abnormal results mean | Abnormal results may show Down syndrome (trisomy 21 = three copies of chromosome 21 instead of the normal two copies), Trisomy 18, Philadelphia chromosome, Klinefelter syndrome , Turner syndrome , or other abnormalities. Additional conditions under which the test may be performed:chronic myelogenous leukemia (CML) or other leukemias multiple birth defects a baby born with genitals that are neither completely male or female |
| What the risks are | The risks are related to the procedure used to obtain the specimen. There is a specialized kind of risk in that an abnormal result may have occurred during growth of the cells after they left the body. For this reason it is often prudent to repeat the karyotype test to make sure an abnormal chromosome constitution in the body of the patient. There is a rare difference between the apparent sex of the patient and their chromosomes. For example, a baby may look like it has a penis and be called a boy but turn out to have the chromosomes of a girl. This can raise issues of what gender to raise the child. |
| Special considerations | Chemotherapy may cause chromosome breaks interfering with normal results. Mixtures of 2 different populations of cells or chromosome constititutions are sometimes observed. This is called mosaicism and is more common in chorionic villous sampling or amniocentesis. It does not necessarily mean that the baby has abnormal chromosomes. |
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