Karyotyping is a genetic test that examines your chromosomes, the 46 tightly packed bundles of DNA inside nearly every cell in your body. The test captures an image of all your chromosomes arranged by size and shape, making it possible to spot missing, extra, or rearranged ones. It’s used to diagnose genetic conditions, investigate fertility problems, guide cancer treatment, and check a developing fetus for chromosomal disorders.
How the Test Works
Every time a cell divides, its chromosomes briefly condense into compact, visible structures. Karyotyping takes advantage of that moment. A lab technician takes a sample of your cells, places them in a growth medium, and then adds a chemical (colchicine) that freezes cell division right when the chromosomes are most condensed and easiest to see. The cells are then placed on a glass slide, stained, and photographed under a microscope.
The most common staining method, called G-banding, uses an enzyme and a dye to create a unique pattern of light and dark bands on each chromosome. At standard resolution, technicians can distinguish around 400 to 550 bands across all 46 chromosomes, and high-resolution techniques can push that to 850 bands. These banding patterns act like a barcode: if a section is missing, duplicated, or stuck onto a different chromosome, the pattern won’t match what’s expected.
Once stained and photographed, the chromosomes are arranged into numbered pairs from largest to smallest, with the sex chromosomes (X and Y) placed at the end. This organized image is the karyotype itself, and a trained cytogeneticist reviews it for anything unusual.
What Samples Are Used
The sample depends on who’s being tested and why. For most adults and children, a standard blood draw provides enough white blood cells to culture. Newborns sometimes have blood collected from a heel prick or from the umbilical cord. For cancer patients, bone marrow is often the preferred source because it captures the tumor’s own chromosomes directly.
Prenatal testing uses one of two methods. Chorionic villus sampling (CVS) collects a small piece of the placenta and can be done between 10 and 13 weeks of pregnancy. Amniocentesis draws a sample of the fluid surrounding the fetus and is typically performed after 15 weeks. In some cases, skin biopsies or cheek swabs also provide enough cells.
What Karyotyping Can Detect
Chromosome abnormalities fall into two broad categories: numerical and structural. Karyotyping reliably detects both, as long as the change is large enough to see under a microscope (generally larger than about 5 million base pairs of DNA).
Numerical Abnormalities
A normal human cell has 46 chromosomes. When a cell has an extra or missing chromosome, it’s called aneuploidy. Some of the most well-known conditions detected by karyotyping include:
- Down syndrome (trisomy 21): three copies of chromosome 21 instead of two
- Edwards syndrome (trisomy 18): an extra chromosome 18, causing severe developmental problems
- Patau syndrome (trisomy 13): an extra chromosome 13
- Turner syndrome: only one X chromosome instead of two sex chromosomes (45 total)
- Klinefelter syndrome: an extra X chromosome in males (47, XXY)
Structural Abnormalities
Sometimes the total number of chromosomes is normal, but pieces have broken off, flipped around, or reattached in the wrong place. Karyotyping can identify deletions (missing segments), duplications (extra copies of a segment), inversions (a segment flipped backward), translocations (a segment moved to a different chromosome), and ring chromosomes (where both ends of a chromosome fuse together). One particularly important type, called a Robertsonian translocation, involves two chromosomes fusing near their centers and is a common cause of inherited Down syndrome in families.
Why Doctors Order the Test
For adults, the most common reasons are unexplained infertility, repeated miscarriages, or stillbirths. A hidden chromosomal rearrangement in one or both parents can cause pregnancies to fail even when both parents appear healthy. Karyotyping can reveal whether a balanced translocation or other structural change is the underlying cause.
In cancer care, karyotyping plays a direct role in treatment decisions. Certain blood cancers, especially leukemias and lymphomas, produce characteristic chromosome changes. In acute myeloid leukemia, for example, the complexity of the karyotype (how many chromosomal changes are present) helps predict how aggressive the disease is and which treatment approach is most appropriate. Some specific translocations place patients in favorable or intermediate risk categories, while highly complex karyotypes signal a worse prognosis.
Prenatal karyotyping is offered when screening tests suggest an elevated risk of a chromosomal condition, when an ultrasound shows structural abnormalities, when either parent carries a known chromosomal rearrangement, or when the birthing parent is over 35. It can also help explain a late pregnancy loss or stillbirth.
Infants and young children may be tested if they show signs of a genetic syndrome, such as unusual physical features, developmental delays, or ambiguous genitalia.
How Long Results Take
Because the cells need to be cultured and allowed to divide before they can be analyzed, karyotyping is not instant. Results from a blood sample typically take one to two weeks. Prenatal samples from amniocentesis or CVS can take a similar timeframe, sometimes slightly longer if the cells are slow to grow in culture. Bone marrow samples for cancer patients are often prioritized and may return faster, but turnaround still depends on cell growth.
Karyotyping vs. Chromosomal Microarray
Karyotyping has been the gold standard for chromosome analysis for decades, but newer technology called chromosomal microarray (CMA) offers higher resolution. In a study of 487 prenatal samples, CMA detected chromosomal abnormalities in about 13% of cases compared to 9% for standard karyotyping, a difference of roughly 4 percentage points. The gap comes from CMA’s ability to spot tiny deletions and duplications smaller than 3 to 5 million base pairs, changes too small to see under a microscope.
That doesn’t make karyotyping obsolete. CMA cannot detect balanced structural rearrangements like translocations and inversions, because no genetic material is gained or lost in those cases. It also struggles with low-level mosaicism, where only some cells carry an abnormality. In one comparison, karyotyping caught balanced translocations, inversions, and certain types of mosaicism that CMA missed entirely. For this reason, the two tests are often complementary. CMA is now recommended as a first-line test for children with developmental delays or autism spectrum disorders, while karyotyping remains essential for evaluating fertility issues, cancer, and balanced rearrangements.
What a Normal Result Looks Like
A normal female karyotype is written as 46,XX and a normal male as 46,XY. The first number represents the total chromosome count, and the letters indicate the sex chromosomes. Any deviation from this notation signals a finding. For instance, 47,XY,+21 means a male with an extra copy of chromosome 21 (Down syndrome), while 45,X means a female with only one X chromosome (Turner syndrome). This shorthand follows an internationally standardized naming system that was most recently updated in September 2024, ensuring labs around the world describe the same findings in the same way.