A karyotype is a cytogenetic test that examines the number and large-scale structure of chromosomes under a microscope. Chromosomes are visualised using a banding method, arranged in pairs and assessed for numerical or structural differences.
What can a karyotype detect?
The test may identify a missing or extra chromosome (aneuploidy), large deletions or duplications, translocations, inversions and some forms of mosaicism. Balanced chromosome rearrangements usually do not cause a net loss or gain of genetic material and may cause no symptoms in the carrier, but they can be relevant to reproduction.
When may the test be requested?
- Suspected chromosome syndrome or congenital differences: when clinical findings suggest a numerical or structural chromosome change.
- Reproductive concerns: in selected infertility cases, recurrent pregnancy loss or after a structural chromosome change is identified in pregnancy-loss tissue.
- Prenatal diagnosis: on CVS or amniotic-fluid samples after a high-risk screen, relevant ultrasound finding or known familial chromosome rearrangement.
- Sex chromosome conditions: when a condition such as Turner or Klinefelter syndrome is suspected.
- Haematological disease: under disease-specific cytogenetic protocols for the diagnosis, classification or monitoring of certain leukaemias and bone-marrow disorders.
Which samples can be used?
Depending on the clinical question, peripheral blood, bone marrow, amniotic fluid, chorionic villi or pregnancy-loss tissue may be used. Conventional karyotyping generally requires a culture of living, dividing cells. Correct collection tube, transport conditions and timely delivery are therefore important. Preparation and turnaround time vary by sample and laboratory protocol.
What are the limitations?
A karyotype does not sequence individual genes and may miss DNA losses or gains below its resolution. Chromosomal microarray may be more suitable for small copy-number changes, FISH for a defined region, and molecular testing for single-gene variants.
Microarray detects smaller unbalanced changes but generally does not detect balanced translocations or inversions. A newer method therefore does not replace karyotyping for every clinical question; method selection must match the suspected change.
What does a normal result mean?
A normal karyotype means that no numerical or large structural chromosome difference was seen within the resolution of the method in the cells examined. It does not exclude every genetic disorder. When mosaicism is suspected, the number of cells and the tissue tested may influence detection.
Select the cytogenetic method that answers the clinical question
For information about karyotyping, sample requirements and complementary methods such as FISH or microarray, explore our Cytogenetics service.
