Chromosome Analysis (Karyotype)

What is chromosome analysis?

Chromosomes are structures in the cell nucleus that carry genetic information. Humans normally have 23 pairs, that is, 46 chromosomes; one of these pairs is the X and Y chromosomes, which determine sex. Chromosome analysis, or karyotyping, is the examination of the number and structure of these chromosomes under a microscope. Numerical changes, such as an extra or a missing chromosome, and structural changes, such as chromosome segments changing place, breaking off or being inverted, can be assessed with this method.

In a peripheral blood karyotype, white blood cells called lymphocytes are grown for a period in a special medium in the laboratory. The cells are stopped at the stage of division in which the chromosomes can be seen clearly, they are stained, and the chromosomes are arranged and examined according to their banding patterns. The culture and examination stages form a meticulous process that relies on manual work. This method shows large changes at the chromosome level; however, very small deletions and duplications, as well as single-gene changes, cannot be detected with a karyotype.

When is chromosome analysis ordered?

Chromosome analysis is ordered on a doctor's recommendation when symptoms or medical history suggest a chromosomal abnormality. In recurrent pregnancy loss, one of the partners may have a balanced chromosomal change that causes them no symptoms; for this reason, the test is often performed on both partners together. In infertility investigations, assessing chromosome structure in both women and men is also an important step.

• Recurrent pregnancy loss (miscarriage)
• Female or male infertility, a very low sperm count or no sperm at all
• Delayed puberty or absence of menstrual periods
• Developmental delay, learning difficulties or congenital anomalies in children
• Assessment of a suspected chromosomal syndrome such as Down syndrome
• A known chromosomal change in the family

This test is performed on the person's own blood sample; it is not used for prenatal diagnosis of the baby during pregnancy. Assessing the baby's chromosomes requires different samples and methods. Your doctor decides whether the test is right for you, taking into account your medical history and previous tests.

How do you prepare for chromosome analysis?

No fasting is needed for chromosome analysis; the blood sample can be given at any time of day. Because the analysis relies on growing live cells in the laboratory, the sample is collected in a special tube containing heparin and delivered to the laboratory without delay. For this reason, the appointment time is planned so that the sample can reach the laboratory under appropriate conditions.

If you have recently received a blood transfusion, had chemotherapy or undergone a bone marrow transplant, be sure to mention this; these can affect the result. Genetic tests are performed with informed consent, and genetic counseling before the test is recommended. Our nurse collects your blood sample at your home or workplace; the sample is analyzed at an accredited laboratory.

How are chromosome analysis results interpreted?

The result report gives the chromosome number and the sex chromosomes in an international notation; for example, a normal male karyotype is written as 46,XY and a normal female karyotype as 46,XX. If a numerical or structural change is detected, its type and the chromosome region involved are described in the report. In some people, the change may be seen in only some of the cells; this is called mosaicism.

A normal karyotype shows that no large change was detected at the chromosome level; however, it does not rule out all genetic causes. If considered necessary, more detailed tests such as microarray may be planned. For people found to have a balanced change, the options for future pregnancies are discussed in detail in genetic counseling. Your result should be evaluated together with your medical history and other findings by your doctor and, if needed, a medical geneticist.