What Is a Karyotype Test and How Is it Done?

Karyotypes Can Diagnose a Number of Medical Conditions before or after Birth

Human karyotype. Credit: PASIEKA/SPL / Getty Images

A karyotype is a photograph of the chromosomes in a cell. Karyotypes can be taken from blood cells, fetal skin cells (from amniotic fluid or the placenta), and bone marrow cells. Karyotypes can be used to confirm Down syndrome, which is caused by an extra copy of chromosome number 21. 

From a patient’s perspective, a karyotype is a simple blood test. But what happens to the blood after it is collected is actually quite complex.

This step by step guide will help you understand why it takes up to a week to get results.

1. Sample Collection

The first step in performing a karyotype is to collect a sample. In newborns, a blood sample containing red blood cells, white blood cells, serum, and other fluids is collected. A karyotype will be done on the white blood cells which are actively dividing (a state known as mitosis). During pregnancy, the sample can either be amniotic fluid collected during an amniocentesis or a piece of the placenta collected during a chorionic villi sampling test (CVS). The amniotic fluid contains fetal skin cells which are used to generate a karyotype.

2. Transport To the Laboratory

Karyotypes are performed in a specific laboratory called a cytogenetics lab –– a lab which studies chromosomes. Not all hospitals have cytogenetics labs. If your hospital or medical facility doesn’t have its own cytogenetics laboratory, the test sample will be sent to a lab that specializes in karyotype analysis.

The test sample is analyzed by specially trained cytogenetic technologists, Ph.D cytogeneticists, or medical geneticists.

3. Separating the Cells

In order to analyze chromosomes, the sample must contain cells that are actively dividing. In blood, the white blood cells actively divide. Most fetal cells actively divide as well.

Once the sample reaches the cytogenetics lab, the non-divided cells are separated from the dividing cells using special chemicals.

4. Growing Cells

In order to have enough cells to analyze, the dividing cells are grown in special media or a cell culture. This media contains chemicals and hormones that enable the cells to divide and multiply. This process of culturing can take three to four days for blood cells, and up to a week for fetal cells.

5. Synchronizing Cells

Chromosomes are a long string of human DNA. In order to see chromosomes under a microscope, chromosomes have to be in their most compact form in a phase of cell division (mitosis) known as metaphase. In order to get all the cells to this specific stage of cell division, the cells are treated with a chemical which stops cell division at the point where the chromosomes are the most compact.

6. Releasing the Chromosomes From Their Cells

In order to see these compact chromosomes under a microscope, the chromosomes have to be out of the white blood cells. This is done by treating the white blood cells with a special solution that causes them to burst. This is done while the cells are on a microscopic slide. The leftover debris from the white blood cells is washed away, leaving the chromosomes stuck to the slide.

7. Staining the Chromosomes

Chromosomes are naturally colorless. In order to tell one chromosome from another, a special dye called Giemsa dye is applied to the slide. Giemsa dye stains regions of chromosomes that are rich in the bases adenine (A) and thymine (T). When stained, the chromosomes look like strings with light and dark bands. Each chromosome has a specific pattern of light and dark bands which enables cytogeneticist to tell one chromosome from another. Each dark or light band encompasses hundreds of different genes.

8. Analysis

Once chromosomes are stained, the slide is put under the microscope for analysis.

A picture is then taken of the chromosomes. By the end of the analysis, the total number of chromosomes will be determined and the chromosomes arranged by size.

9. Counting Chromosomes

The first step of the analysis is counting the chromosomes. Most humans have 46 chromosomes. People with Down syndrome have 47 chromosomes. It is also possible for people to have missing chromosomes or more than one extra chromosome. By looking at just the number of chromosomes, it is possible to diagnose different conditions including Down syndrome. 

10. Sorting Chromosomes

After determining the number of chromosomes, the cytogeneticist will start sorting the chromosomes. To sort the chromosomes, a cytogeneticist will compare chromosome length, the placement of centromeres (the areas where the two chromatids are joined), and the location and sizes of G-bands. The chromosomes pairs are numbered from largest (number 1) to smallest (number 22). There are 22 pairs of chromosomes, called autosomes, which match up exactly. There are also the sex chromosomes, females have two X chromosomes while males have an X and a Y.

11. Looking at the Structure

In addition to looking at the total number of chromosomes and the sex chromosomes, the cytogeneticist will also look at the structure of the specific chromosomes to make sure that there is no missing or additional material as well as structural abnormalities like translocations.

12. The Final Result

In the end, the final karyotype shows the total number of chromosomes, the sex, and any structural abnormalities with individual chromosomes. A digital picture of the chromosomes is generated with all of the chromosomes arranged by number.

When diagnosing Down syndrome, the focus tends to be on the total number of chromosomes, but a karyotype gives you much more information than that. 

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