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Diagnosis and tests

Karyotype

The related article (“Cytogenetics: karyotype and FISH”) explains the blood count. This topic looks closely at the classic cytogenetic test, the karyotype, and at how its written result is read.

Last updated: ~1 min read
Pairs of chromosomes in rows.
Figure 1.A banded karyogram of chromosomes.

01Short answer

A karyotype is a “picture” of all the chromosomes in a cell. Bone marrow cells are grown in the lab for a day or two, stopped in the middle of dividing, spread onto a slide and coloured with banding stains. A specialist usually looks at at least 20 cells and arranges the chromosomes in pairs by size (a karyogram). The result is written as a special formula: first the number of chromosomes (normally 46), then the sex chromosomes (XX or XY), then any changes, for example t(9;22) for a translocation, del(5q) for a deletion, or +8 for an extra chromosome. The strength of a karyotype is that it gives an overview of the whole genome; its weaknesses are that it misses very small changes and needs living, dividing cells.

02Simple explanation

A picture of all the chromosomes.

At least 20 cells.

A formula: number, sex, changes.

03Scientific explanation

Resolution: a banded karyotype sees changes of roughly millions of base pairs or more; smaller ones need FISH or molecular tests.

Clone: a change counts as clonal if it is seen in at least two cells (or three cells for a loss).

04What happens in the body?

A karyogram

Chromosomes are lined up in pairs and each one is identified.

Bands

Light and dark bands help find the changed segment.

Two chromosomes with a glowing segment.
Figure 2.A changed band pattern.

20 cells

Looking at many cells shows the change is not a one-off.

A grid of twenty cells.
Figure 3.Twenty cells are examined.

05How a karyotype is made

Four steps.

Growing — Cells multiply.1GrowingCells multiply.
Spreading — Chromosomes onto a slide.2SpreadingChromosomes onto a slide.
Staining — Bands appear.3StainingBands appear.
Karyogram — Pairs put in order.4KaryogramPairs put in order.

06Reading the formula

PartMeaning
46Number of chromosomes
XX / XYSex chromosomes
t(9;22)A translocation between chromosomes 9 and 22
del(5q)Loss of the long arm of chromosome 5
+8An extra chromosome 8
[20]Number of cells examined

07Key terms

Karyotype
The full picture of the chromosomes.
Karyogram
An image of chromosomes arranged in pairs.
Band
The pattern seen after staining.
Clone
A group of cells with the same change.
p and q
The short and long arms of a chromosome.

08Key points

  • A karyotype looks at all the chromosomes.
  • It needs living, dividing cells.
  • At least 20 cells are assessed.
  • The formula follows set rules.
  • It misses very small changes.
  • FISH and molecular tests complement it.

09Frequently asked questions

When will the result be ready?

Usually in one to three weeks.

What does “the cells did not divide” mean?

Sometimes a sample fails; your doctor may choose FISH or a repeat test.

Is the karyotype repeated?

Sometimes, to assess response to treatment or relapse.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. When will the result be ready?

  2. What does “the cells did not divide” mean?

  3. Is the karyotype repeated?

My questions

12Sources

  1. National Cancer Institute (NCI). Definition: cytogenetic analysis
  2. MedlinePlus Genetics. What is a chromosome?
  3. National Cancer Institute (NCI). CML treatment (PDQ), patient version
  4. ESMO. ESMO Patient Guides
  5. American Cancer Society. Tests for Acute Myeloid Leukemia (AML)
  6. MedlinePlus (U.S. National Library of Medicine). Comprehensive Metabolic Panel (CMP)
  7. MedlinePlus (U.S. National Library of Medicine). Liver Function Tests
  8. MedlinePlus (U.S. National Library of Medicine). Electrolyte Panel
  9. Leukemia (journal), 2022. Khoury JD et al. The 5th edition of the WHO Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/Dendritic Neoplasms
  10. Leukemia (journal), 2022. Alaggio R et al. The 5th edition of the WHO Classification of Haematolymphoid Tumours: Lymphoid Neoplasms
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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