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Diagnosis and tests

Looking at cells under the microscope

The related article (“Blood smear”) lists the steps of diagnosis. This topic looks closely at one step: how blood and bone marrow cells are studied under the microscope.

Last updated: ~1 min read
A slide under a lens.
Figure 1.A stained film under the microscope.

01Short answer

Microscopy is the oldest and still a central method for diagnosing leukaemia. A drop of blood or bone marrow is spread thinly on a glass slide and coloured with special stains, and a specialist (a haematologist or pathologist) counts and describes the cells one by one. They look at the size of the cells, the shape of the nucleus, the colour of the cytoplasm and any granules. The percentage of blasts is especially important: a diagnosis of acute leukaemia is often based on it. Some features point to a type, such as Auer rods in AML or “hairy” projections in hairy cell leukaemia. Cytochemical stains can help tell myeloid and lymphoid cells apart.

02Simple explanation

A thin film, stain, microscope.

The blast percentage: a key number.

How cells look hints at the type.

03Scientific explanation

Morphology: the study of cell shape and structure; the foundation of diagnosis, but it does not confirm the type on its own.

Cytochemistry: stains such as myeloperoxidase help tell myeloid blasts from lymphoid blasts.

04What happens in the body?

A stained film

Stains colour the nucleus purple and the cytoplasm pink-blue.

A blast and a mature cell

A blast has a large nucleus and thin cytoplasm; a mature neutrophil has a segmented nucleus.

A blast and a neutrophil side by side.
Figure 2.A blast and a mature cell.

An Auer rod

A needle-like rod inside a blast points to myeloid origin.

A blast with a pink rod.
Figure 3.An Auer rod in a blast.

05How microscopy is done

Four steps.

Drop — Sample on a slide.1DropSample on a slide.
Film — Spread thinly.2FilmSpread thinly.
Stain — Coloured.3StainColoured.
Reading — Cells counted.4ReadingCells counted.

06What is looked for under the microscope

FeatureWhat it suggests
Blast percentageAcute leukaemia
Shape of the nucleusHow mature the cell is
Auer rodsMyeloid origin (AML)
Hairy projectionsHairy cell leukaemia
Small mature lymphocytes, “smudge” cellsCLL

07Key terms

Morphology
The study of cell shape.
Smear
A thin layer of cells on glass.
Blast
An immature cell.
Auer rod
A needle-like body in a myeloid blast.
Cytochemistry
Stains that show substances in cells.

08Key points

  • Microscopy is the foundation of diagnosis.
  • The blast percentage matters.
  • Some features point to a type.
  • It is not enough on its own.
  • It is read together with other tests.
  • A specialist assesses cells one by one.

09Frequently asked questions

How quickly is the result ready?

Microscopy is usually the fastest test, sometimes the same day.

Why are more tests needed?

Some types look alike under the microscope; flow cytometry and genetics separate them (“Flow cytometry”).

The numbers are confusing; what now?

Ask your doctor to explain (“Reading your results with your doctor”).

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. How quickly is the result ready?

  2. Why are more tests needed?

  3. The numbers are confusing; what now?

My questions

12Sources

  1. MedlinePlus. Blood smear
  2. MedlinePlus. Bone marrow aspiration
  3. National Cancer Institute (NCI). Adult AML treatment (PDQ), patient version
  4. National Cancer Institute (NCI). Leukemia — Patient Version
  5. National Cancer Institute (NCI). Adult Acute Lymphoblastic Leukemia Treatment (PDQ®) — Patient Version
  6. National Cancer Institute (NCI). Chronic Myeloid Leukemia Treatment (PDQ®) — Patient Version
  7. National Cancer Institute (NCI). Childhood Acute Lymphoblastic Leukemia Treatment (PDQ®) — Patient Version
  8. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: minimal residual disease
  9. MedlinePlus (U.S. National Library of Medicine). Leukemia
  10. MedlinePlus. Complete Blood Count (CBC)
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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