A karyotype, also called a chromosome test or chromosome analysis, is a test that looks for differences in the size, structure, or number of chromosomes in a sample of cells.

A karyotype may be done for several reasons, including:

• to confirm a clinical diagnosis of Down syndrome or other chromosome-related genetic syndromes
• if you have a family history of a chromosome rearrangement
• if a child or adult has symptoms that may be due to a chromosome condition, such as developmental delay, intellectual disability, or a congenital anomaly (Another test called a chromosomal microarray is generally preferred over a karyotype in this situation.)
• if there are concerns with the fetus during a pregnancy that may suggest a chromosome condition
• if a couple has multiple miscarriages (pregnancy loss), stillbirth, or fertility issues
• to help determine the best treatment for certain cancers or blood disorders that involve changes in chromosomes

A karyotype is most commonly done on a blood sample. It can give 4 kinds of results:

• Positive: The test found a difference in the number or structure of the chromosomes. This type of result can mean different things about your health or your child’s health depending on the specific chromosome variation. A referral to a genetics professional is recommended to get information specific to your result.
• Negative: The test did not find any differences in the number or structure of the 46 chromosomes. There could still be a genetic concern, but the karyotype was not able to detect it.
• Uncertain: The test found a difference in the number or structure of the chromosomes, but it is not well understood. The difference may or may not explain your or your child’s health or learning concerns. Your healthcare team will discuss possible next steps to try to better understand this result.
• Incidental: The test found a difference in the number or structure of the chromosomes, but it is not related to why the test was done. For example, the test may be ordered to confirm a clinical diagnosis of Down syndrome but may show that there is an unexpected number of X or Y chromosomes.

Because the karyotype only looks at the number and structure of the chromosomes, many genetic causes for health concerns cannot be found by this test. In some cases, your healthcare team might suggest a more detailed test to look at your genes, such as a chromosomal microarray or gene panel test.