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Diagnosis and tests

Flow cytometry

The related article (“Flow cytometry (immunophenotyping)”) explains blood tests. This topic looks closely at one of the main tests in diagnosing leukaemia, flow cytometry: how it works and what it shows.

Last updated: ~1 min read
Cells in a channel and a laser beam.
Figure 1.Cells pass in front of a laser.

01Short answer

Flow cytometry uses a machine that rapidly counts cells one at a time and recognises particular proteins (markers) on their surface and inside them. Antibodies labelled with fluorescent dyes are added to a blood or bone marrow sample; each antibody binds to its own marker. The cells then pass one by one through a thin channel past a laser beam, and the machine records each cell’s size, structure and the colours it gives off. The thousands of cells are sorted into groups, and an abnormal population stands out. This shows whether the leukaemia is myeloid or lymphoid, and B-cell or T-cell. Flow cytometry is also used to measure remaining disease (MRD) after treatment.

02Simple explanation

Cells one by one pass a laser.

Coloured antibodies recognise markers.

Groups reveal abnormal cells.

03Scientific explanation

CD markers: proteins on the cell surface are named with “CD” numbers (for example CD19 for B cells, CD3 for T cells, CD33 for myeloid cells).

Sensitivity: multicolour flow cytometry can find one leukaemia cell among 10,000 normal cells, which matters for MRD.

04What happens in the body?

In front of the laser

As each cell passes the laser, it scatters light and gives a coloured signal.

Colour tags

Antibodies bind to cell markers and make them “light up”.

Cells with coloured tags.
Figure 2.Coloured antibody tags.

A cluster plot

Abnormal cells appear on the plot as a separate group.

Coloured clusters of dots.
Figure 3.An abnormal group on the plot.

05Steps of flow cytometry

Four steps.

Sample — Blood or marrow.1SampleBlood or marrow.
Tagging — Coloured antibodies.2TaggingColoured antibodies.
Laser — Read one by one.3LaserRead one by one.
Groups — Analysed.4GroupsAnalysed.

06What flow cytometry tells us

QuestionExample answer
Myeloid or lymphoid?AML or ALL
B or T?B-ALL or T-ALL
Mature or immature?CLL or acute leukaemia
Anything left after treatment?MRD measurement
Is a target marker present?Choosing some antibody treatments

07Key terms

Flow cytometry
A method that analyses cells one at a time.
Marker
A protein “tag” on a cell.
CD
The naming system for markers.
Fluorescence
A dye glowing under light.
MRD
Minimal residual disease.

08Key points

  • Flow cytometry counts cells one by one.
  • Markers show the type.
  • It separates myeloid from lymphoid.
  • It separates B from T.
  • It measures MRD.
  • Results are usually ready in 1–2 days.

09Frequently asked questions

Does it hurt?

The test is done in the lab; you only give an ordinary blood or marrow sample.

Why both blood and marrow?

Marrow gives a more accurate picture; blood is convenient for follow-up.

What does a target marker mean?

Some treatments aim at a particular marker; flow cytometry shows whether it is there (“How flow cytometry shows the type”).

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. Does it hurt?

  2. Why both blood and marrow?

  3. What does a target marker mean?

My questions

12Sources

  1. National Cancer Institute (NCI). Definition: flow cytometry
  2. MedlinePlus. Bone marrow tests
  3. National Cancer Institute (NCI). Adult ALL treatment (PDQ), patient version
  4. National Cancer Institute (NCI). Adult Acute Myeloid Leukemia Treatment (PDQ®) — Patient Version
  5. MedlinePlus (U.S. National Library of Medicine). Bone marrow aspiration
  6. National Cancer Institute (NCI). The Genetics of Cancer
  7. National Cancer Institute (NCI). Biomarker Testing for Cancer Treatment
  8. National Cancer Institute (NCI). Questions to Ask Your Doctor about Your Diagnosis
  9. National Cancer Institute (NCI). Feelings and Cancer
  10. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: blast
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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