What this test measures
A G-banded karyotype is a microscopic analysis of all 46 chromosomes from cultured peripheral blood lymphocytes. Blood is collected in a heparin tube, lymphocytes are stimulated to divide (using phytohaemagglutinin), arrested in metaphase, stained with Giemsa to produce characteristic light-and-dark band patterns ("G-bands"), and photographed. A cytogeneticist then arranges the chromosomes by size and centromere position into a karyogram and looks for numerical changes (trisomy, monosomy) or structural changes (translocations, deletions, duplications, inversions, ring chromosomes) at a resolution of ~5–10 megabases.
Results are reported using ISCN 2020 nomenclature, e.g. 47,XY,+21 for Down syndrome or 46,XX,t(9;22)(q34;q11) for the Philadelphia translocation.
Why it matters
Karyotyping remains the foundational test for several clinical questions in India:
• Suspected chromosomal syndromes — Down syndrome (Trisomy 21), Edwards (Trisomy 18), Patau (Trisomy 13), Turner (45,X), Klinefelter (47,XXY) • Recurrent pregnancy loss / parental karyotyping for balanced translocations • Primary or secondary amenorrhoea, infertility workup, ambiguous genitalia • Haematological malignancy diagnosis and risk stratification (e.g. Philadelphia chromosome in CML, t(8;21) in AML) • Pre-implantation parental screening before assisted reproduction
While newer techniques (chromosomal microarray, FISH, sequencing) detect submicroscopic abnormalities, G-banded karyotype is still the standard first-line test for whole-chromosome events and balanced translocations.
How to prepare
No fasting required. 3–5 mL of blood is collected into a sodium-heparin (green-top) tube — EDTA tubes do not work because they inhibit cell culture. Send to the lab within 24 hours at room temperature (do not refrigerate). Mention: indication for testing, suspected diagnosis, previous chromosomal results in the family, fertility history, recent chemotherapy or radiation. Recent transfusion can confuse results — ideally wait 3 months after transfusion before sampling.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Karyotype Result (—)[1][2] | 46,XX (female) or 46,XY (male) | Normal karyotype does not exclude submicroscopic abnormalities, single-gene disorders, or low-level mosaicism — consider chromosomal microarray, FISH, or targeted sequencing if clinically indicated. | Abnormal karyotypes vary widely: trisomies (e.g. 47,XY,+21 = Down), monosomies (e.g. 45,X = Turner), translocations (e.g. t(9;22) = Philadelphia in CML), deletions, duplications, inversions, marker chromosomes. Each requires a specific clinical interpretation by a geneticist. |
Common karyotype findings and clinical context
| Karyotype | Condition | Typical presentation |
|---|---|---|
| 47,XX or XY,+21 | Down syndrome (Trisomy 21) | Intellectual disability, characteristic facies, cardiac anomalies |
| 47,XX or XY,+18 | Edwards syndrome (Trisomy 18) | Severe developmental delay, organ anomalies, often lethal in infancy |
| 45,X | Turner syndrome | Short stature, gonadal dysgenesis, primary amenorrhoea |
| 47,XXY | Klinefelter syndrome | Tall stature, infertility, gynaecomastia |
| 46,XX,t(9;22)(q34;q11) | Philadelphia chromosome — CML | Chronic myeloid leukemia |
| Balanced translocation (parental) | Carrier — risk of recurrent miscarriage / abnormal offspring | Often phenotypically normal |
Frequently asked questions
When is karyotyping ordered?
Common indications: suspected chromosomal syndrome (Down, Turner, Klinefelter), recurrent pregnancy loss (parental karyotyping), primary amenorrhoea, infertility, ambiguous genitalia, severe developmental delay, and certain haematological cancers (leukemia, MDS).
How long does the report take?
Because cells need to be cultured for several days before analysis, blood karyotyping typically takes 10–21 days. Urgent fetal / neonatal karyotypes can be expedited but still take days, not hours.
Is fasting required?
No. Eat and drink normally.
How is karyotyping different from chromosomal microarray (CMA)?
Karyotyping detects whole-chromosome abnormalities and large structural rearrangements (~5–10 Mb resolution). CMA detects much smaller copy-number variants (1–100 kb) but misses balanced translocations and low-level mosaicism. The two are complementary.
Can karyotyping detect Down syndrome before birth?
Yes — fetal karyotype from amniotic fluid (amniocentesis) or chorionic villus sampling. Non-invasive prenatal testing (NIPT) is also widely used as a screen, with karyotype confirming positive results.
Why do I need to be off recent transfusion before testing?
Transfused lymphocytes can contaminate the culture and show donor karyotype, leading to confusing results. Wait at least 3 months after transfusion.
My karyotype is normal but my doctor still suspects a genetic disorder — what next?
A normal karyotype does not rule out submicroscopic deletions, duplications, single-gene disorders, or low-level mosaicism. Consider chromosomal microarray (CMA), FISH for specific deletions, or targeted gene panel / whole-exome sequencing.
Related Molecular / Genetic tests
Tests commonly ordered alongside Karyotyping by G-Banding Blood, or that help interpret an unexpected result.
Sources & references
- NIH MedlinePlus — Karyotype Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Karyotyping · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — Chromosome Analysis · accessed 2026-05-30T00:00:00.000Z
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