Bone Marrow Examination
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Direct answer
Bone marrow examination samples the marrow by aspiration, for cytology, flow cytometry and cytogenetics, and by trephine biopsy, for cellularity, architecture and fibrosis. The two procedures are complementary rather than interchangeable, since the aspirate shows cell morphology and the core shows the marrow pattern. In adults, the posterior superior iliac spine is the usual site, and indications include unexplained cytopenias, suspected leukaemia or lymphoma and fever of unknown origin.
What you must remember
- Aspiration: liquid marrow for morphology, cytochemistry, flow cytometry, cytogenetics and molecular tests, and the source of the differential count.
- Trephine biopsy: an intact core for cellularity, architecture, fibrosis, granulomas and infiltration, essential when the aspirate fails.
- Dry tap: an aspirate yielding no marrow, typical of myelofibrosis, hairy cell leukaemia or heavy marrow fibrosis, which needs a trephine.
- Site: the posterior superior iliac spine in adults, with the anterior iliac crest as an alternative and the sternum avoided in routine practice.
- Peripheral dilution: blood contamination lowers marrow cellularity, so the particles and trephine must be checked before the marrow is called hypocellular.
- Iron stain: Prussian blue stain on the aspirate shows iron stores and ring sideroblasts in sideroblastic anaemia.
- Auer rods: needle-shaped inclusions in myeloblasts, which point strongly to acute myeloid leukaemia.
- Pre-procedure check: platelet count and coagulation status are reviewed before the procedure, as bleeding is the main risk.
Matching the marrow to the disease
A 45-year-old woman has pancytopenia, teardrop cells on the peripheral smear and a dry tap on aspiration. A dry tap in this setting shifts the examination towards the trephine, because the core shows the fibrosis that the aspirate cannot.
| Condition | Marrow finding | Exam clue |
|---|---|---|
| Acute myeloid leukaemia | 20 per cent or more blasts | Auer rods in myeloblasts |
| Gaucher disease | Lipid-laden macrophages | Crumpled tissue paper cytoplasm |
| Myelofibrosis | Dry tap, fibrosis on trephine | Teardrop cells on smear |
| Aplastic anaemia | Hypocellular marrow | Fat spaces with few cells |
| Tuberculosis | Caseating granulomas | Culture and staining needed |
| Megaloblastic anaemia | Megaloblastic erythroid precursors | Giant metamyelocytes |
The table lists the classic finding for each condition, and the trephine usually confirms what the aspirate suggests. Where the aspirate and core disagree, the core governs the architecture and the aspirate governs the morphology. A dry tap is therefore a finding to report and act on, not a failed procedure to simply repeat.
Where the exam tries to catch you
Most lost marks come from taking a procedural detail for a diagnosis, or from misreading a dry tap.
- Trap: Relying on the aspirate alone after a dry tap. Correct: A dry tap usually means fibrosis or hypocellularity, so a trephine biopsy is needed to assess the marrow.
- Trap: Taking the sternum for routine aspiration in adults. Correct: The posterior superior iliac spine is the standard site, because sternal puncture carries a risk of mediastinal injury.
- Trap: Calling a blood-diluted aspirate hypocellular. Correct: Peripheral blood dilution lowers cellularity, so the particles and the trephine must be assessed first.
- Trap: Mistaking Gaucher cells for leukaemic blasts. Correct: Gaucher cells are lipid-laden macrophages with crumpled tissue paper cytoplasm, not blasts.
Frequently asked questions
Which site is used for bone marrow aspiration in adults?
The posterior superior iliac spine is the standard site for bone marrow aspiration and trephine biopsy in adults. The anterior iliac crest is an alternative, while the sternum is avoided in routine practice because of the risk of mediastinal injury.
When is trephine biopsy preferred over aspiration alone?
Trephine biopsy is preferred after a dry tap or when marrow architecture, cellularity or fibrosis must be assessed. It is also needed for lymphoma infiltration, granulomas and myeloproliferative neoplasms, where the core shows what the aspirate cannot.
What finding suggests myelofibrosis on bone marrow examination?
A dry tap with fibrosis on trephine biopsy suggests myelofibrosis, particularly with teardrop cells and a leucoerythroblastic picture on the peripheral smear. The diagnosis is confirmed on the trephine rather than on the aspirate alone.
Which special stains are used on a bone marrow aspirate?
Prussian blue iron stain assesses iron stores and identifies ring sideroblasts, while myeloperoxidase cytochemistry helps classify acute leukaemias. Flow cytometry and cytogenetics add lineage and genetic detail to the morphology.
What are Auer rods and what do they indicate?
Auer rods are needle-shaped cytoplasmic inclusions within myeloblasts, seen in acute myeloid leukaemia. Their presence in a blast population points strongly to a myeloid lineage and helps distinguish acute myeloid leukaemia from acute lymphoblastic leukaemia.