Karyotype Test

General Description

A karyotype is the representation of the set of chromosomes that make up each of a person's cells. In humans, the standard pattern consists of 23 pairs of chromosomes, but variations may occur in cases of genetic abnormalities.

The karyotype test is a common analysis in clinical genetics, used as part of prenatal studies, for diagnosing genetic disorders in children, or for determining the causes of infertility in adults.

To obtain and analyze a person’s genetic map, four types of samples can be collected:

• Peripheral blood karyotype: A blood sample is taken to culture lymphocytes and study the number and structure of their chromosomes. Fetal information can be obtained for prenatal studies through the analysis of a maternal blood sample.
• Bone marrow karyotype: A bone marrow sample is analyzed, obtained via aspiration during a lumbar puncture.
• Amniotic fluid karyotype: A sample is collected during amniocentesis through the pregnant woman’s abdomen.
• Chorionic villus karyotype: This test is conducted on a sample of placental tissue.

Once the laboratory has prepared the sample, the karyotype study can be conducted using different procedures:

• G-banding or Giemsa bands: A staining process is applied to the chromosomes, which adheres to adenine- and thymine-rich regions—two of the hydrogenated bases of DNA. As a result, a characteristic pattern composed of two bands (light and dark) is produced, facilitating the study and analysis of chromosome structure.
• Molecular karyotype (aCGH): More precise than conventional karyotyping, this procedure uses CGH arrays (Comparative Genomic Hybridization) of 60K, 180K, or 400K, offering higher resolution in shorter time frames.
• Spectral karyotype (SKY): This cytogenetic technique detects anomalies more efficiently, as each chromosome is displayed in a different color due to the use of distinct fluorochromes. The analysis combines fluorescence microscopy with imaging systems and automated data analysis methods.

By providing a comprehensive view of the patient’s chromosomes, karyotyping enables the study of both numerical and structural chromosomal abnormalities:

• Numerical abnormalities:
  • Deletion: A chromosome loss occurs (fewer than 46 chromosomes).
  • Duplication: More than one copy of a DNA segment is present.
• Structural abnormalities:
  • Deletion: A portion of chromosomal material is lost.
  • Inversion: A chromosome segment rotates 180 degrees due to a double break that subsequently reattaches, altering its orientation.
  • Translocation: An exchange occurs between two fragments of different chromosomes.
  • • Balanced or reciprocal: The quantity of chromosomal material remains unchanged. Segments are transferred between two chromosomes, altering configuration but not quantity.
    • Unbalanced: There is a gain or loss of chromosomal material.

When is it indicated?

The karyotype test is commonly performed in fertility studies, to determine possible genetic abnormalities in the fetus, or to diagnose diseases in children showing symptoms of chromosomal anomalies. It is also used to investigate the causes of fetal death in late pregnancy or during childbirth (stillbirths) when chromosomal abnormalities are suspected.

Karyotype analysis helps identify the reasons behind recurrent miscarriages or conception difficulties, as well as diagnose conditions such as Down syndrome, Turner syndrome, Klinefelter syndrome, Prader-Willi syndrome, Angelman syndrome, Hodgkin and non-Hodgkin lymphoma, multiple myeloma, or leukemia.

How is it performed?

The procedure varies depending on the type of karyotype being performed:

• Blood karyotype: A blood sample is drawn, usually from the arm.
• Bone marrow karyotype: Local anesthesia is applied to the puncture site. Once it takes effect, a thin needle is inserted between the last two vertebrae of the lumbar region, and a bone marrow sample is extracted using a syringe.
• Amniocentesis or chorionic villus sampling: A small amount of amniotic fluid or placental tissue is extracted through the mother’s abdomen using a fine, hollow needle.

Regardless of sample origin, the laboratory processes them for cell culture to obtain cells in the division process, as chromosomes are only visible during this phase. To allow a detailed study, colchicine is used to halt mitosis (cell division). Subsequently, dyes are applied that color each chromosome part differently based on its composition. This staining allows clear visualization of DNA bands under a microscope, which can be compared with a control sample.

In spectral karyotyping, a different dye is used to color each chromosome distinctly.

In CGH array techniques, the patient’s DNA strand and the reference DNA strand are marked with different colors and structurally altered to form a single strand instead of a double one. Next, probes containing immobilized DNA fragments are added so that both the patient’s and reference DNA hybridize with them. Finally, fluorescence signals are analyzed with software to determine how many sequences belong to the patient and how many belong to the reference genome, identifying possible mutations based on stored data.

Risks

The procedures for collecting samples for karyotype testing may pose certain health risks:

• Blood sample: Some patients may feel dizzy during extraction or develop bruising at the puncture site.
• Bone marrow sample: Dizziness or headaches may occur after collection.
• Amniotic fluid or chorionic villus sample: In rare cases, placental puncture may cause a miscarriage.

What to expect from a karyotype test

Each sample extraction procedure for the karyotype test is different:

• For blood extraction, the patient remains seated or lying on a stretcher with their arm extended. A tourniquet is placed on the upper arm, and the vein is disinfected before inserting the needle to collect the sample using a syringe. It is advisable to apply pressure to the site for a few minutes to prevent bruising.
• For bone marrow extraction, the patient sits slightly leaned forward. Upon needle insertion, pressure is felt, but the anesthesia prevents pain. A bandage should be worn for a few days at the puncture site.
• For amniocentesis or chorionic villus sampling, the patient lies down on a stretcher and must remain as still as possible while the needle is inserted through the abdomen to the placenta. To prevent complications, absolute rest is required for 24 hours and relative rest for the following 48 hours.

Each of these procedures lasts only a few minutes. The test results are available in consultation within one to two weeks after sample collection.

Specialties requesting a karyotype test

Karyotype tests are conducted by geneticists and are typically required in gynecology and obstetrics, assisted reproduction, oncology, and pediatrics.