A karyotype test is a diagnostic lab test that produces a visual map of all 46 of your chromosomes, arranged by size and shape, so doctors can spot missing, extra, or rearranged genetic material. It’s used across medicine, from confirming Down syndrome in a developing fetus to guiding treatment decisions in leukemia. Unlike newer screening tools that estimate risk, a karyotype provides a definitive answer about whether a chromosomal abnormality exists.
How the Test Works
Karyotyping starts with collecting a sample of living cells. The most common source is a simple blood draw, but depending on the situation, cells can also come from bone marrow, amniotic fluid, placental tissue, or a tumor biopsy. These cells are then cultured in a lab for 4 to 7 days, depending on the cell type, in an environment designed to stimulate cell division.
At just the right moment, when the chromosomes are most condensed and visible, a chemical is added to freeze the cells mid-division. The cells are then spread onto glass slides, treated with enzymes, and stained to reveal a distinctive banding pattern on each chromosome. A technician examines these banded chromosomes under a microscope at 1,000x magnification, photographs them, and arranges them into 23 numbered pairs. That organized image is your karyotype.
Because the cells need time to grow in culture, results typically take one to three weeks. Cancer cells, which divide rapidly on their own, sometimes yield faster results since they don’t need as much encouragement to multiply in the lab.
Why Doctors Order a Karyotype
The reasons for karyotyping fall into a few broad categories: prenatal diagnosis, evaluating children with developmental concerns, unexplained infertility, blood cancers, and investigating pregnancy loss.
During pregnancy, a karyotype is the gold standard diagnostic test when there’s elevated risk of a chromosomal condition. This includes pregnancies where the birthing parent is over 35, where a family history of genetic disorders exists, or where an earlier screening test flagged a potential problem. The chromosomes are obtained through amniocentesis (sampling the fluid around the fetus) or chorionic villus sampling (taking a tiny piece of placental tissue). If a pregnancy ends in late loss or stillbirth, karyotyping can reveal whether a chromosomal abnormality was the cause.
In children, the test is ordered when there are signs of a genetic condition, such as developmental delays, unusual physical features, or ambiguous genitalia. In adults, unexplained infertility is one of the most common reasons. Some people carry chromosomal rearrangements that don’t affect their own health but make it difficult to conceive or carry a pregnancy to term. Karyotyping can identify these silent rearrangements.
Conditions a Karyotype Can Detect
The test picks up two main types of problems: wrong numbers of chromosomes and large structural rearrangements.
The most familiar numerical abnormality is Down syndrome, where there are three copies of chromosome 21 instead of two. Other numerical conditions include Edwards syndrome (three copies of chromosome 18), Patau syndrome (three copies of chromosome 13), Turner syndrome (a single X chromosome instead of two sex chromosomes), and Klinefelter syndrome (an extra X chromosome in males, giving an XXY pattern). Rarer findings include cells with entire extra sets of chromosomes, such as 69 or 92 total instead of the usual 46.
Structural abnormalities involve pieces of chromosomes that have broken off and reattached in the wrong place, gone missing, or been duplicated. Some of these are “balanced,” meaning all the genetic material is still present, just rearranged. Balanced rearrangements often cause no symptoms in the person who carries them but can lead to fertility problems or increase the risk of chromosomal conditions in their children. Unbalanced rearrangements, where genetic material is actually gained or lost, tend to cause more obvious health effects.
Karyotyping in Blood Cancers
One of the most important modern uses of karyotyping is in diagnosing and managing blood cancers like leukemia, lymphoma, and multiple myeloma. Cancer cells frequently acquire chromosomal changes as they grow, and identifying those changes helps doctors classify the disease, choose the right treatment, and predict how well that treatment will work.
The most famous example is the Philadelphia chromosome, a specific swap of material between chromosomes 9 and 22 found in certain leukemias. Its discovery was the first proof that cancer could result from a chromosomal abnormality, and it led to the development of targeted therapies. Other characteristic chromosome swaps help distinguish subtypes of lymphoma and leukemia, some of which carry a favorable outlook and others a more aggressive one.
For myeloid cancers like acute myeloid leukemia (AML), karyotyping succeeds in producing a usable result about 88% of the time. Success rates are lower for some other blood cancers. In acute lymphoblastic leukemia, the test works in roughly 70% to 75% of cases, partly because these cancer cells don’t always grow well in culture. In multiple myeloma, the malignant cells rarely divide outside the body, so karyotyping detects an abnormal clone only 10% to 20% of the time, except in advanced disease.
What the Test Cannot See
Karyotyping has a significant resolution limit. Changes to your chromosomes have to be large enough to see under a microscope, which means deletions or duplications smaller than about 5 million base pairs (5 Mb) will be missed. To put that in perspective, 5 Mb is still a tiny fraction of your total DNA, but it’s enormous compared to the single-gene mutations that cause conditions like cystic fibrosis or sickle cell disease. A standard karyotype will not detect those.
For smaller chromosomal changes, doctors turn to higher-resolution tools. Chromosomal microarray analysis can detect copy number changes down to about 27,000 base pairs, roughly 200 times more sensitive than a karyotype. FISH (fluorescence in situ hybridization) uses fluorescent probes to find specific known abnormalities when doctors already have a target in mind. These tests complement karyotyping rather than replacing it, because a karyotype still offers something the others don’t: a complete, cell-by-cell view of every chromosome at once.
Karyotype vs. Prenatal Screening
If you’re pregnant, you may have heard of cell-free DNA screening, sometimes called NIPT (noninvasive prenatal testing). This blood test analyzes fragments of fetal DNA circulating in the mother’s blood and is the most sensitive screening test for common chromosomal conditions like Down syndrome. But screening and diagnosis are not the same thing.
NIPT tells you whether the risk is higher or lower. A karyotype, obtained through amniocentesis or CVS, tells you whether the condition is actually present. The American College of Obstetricians and Gynecologists recommends that all pregnant patients be offered both screening and diagnostic testing options, regardless of age. A positive screening result is typically followed by a diagnostic karyotype to confirm or rule out the finding before any major decisions are made.
What Results Look Like
A normal result is reported as 46,XX (typical female) or 46,XY (typical male). The first number is the total chromosome count, and the letters indicate the sex chromosomes. An abnormal result includes additional notation. For example, 47,XX,+21 means there are 47 chromosomes, two X chromosomes, and an extra copy of chromosome 21, which is Down syndrome. A result of 45,X indicates Turner syndrome: only 45 chromosomes, with a single X and no second sex chromosome.
Structural changes are described with shorthand for the type of rearrangement and the chromosome regions involved. Your doctor or a genetic counselor will walk you through what any abnormal finding means in practical terms, including whether it explains symptoms, affects fertility, or has implications for family members.
Cost and Practical Considerations
The cost of karyotype testing varies widely depending on where you live, which lab processes the sample, and whether insurance covers it. Most insurance plans approve the test only when there’s a specific medical indication, such as a confirmed pregnancy risk factor, signs of a genetic condition, or a cancer diagnosis. If you’re paying out of pocket, it’s worth asking labs directly for their self-pay price, which can be significantly lower than the billed insurance rate. Getting pre-authorization from your insurer before testing can help avoid surprise bills.