Bone marrow aspirate smear

Principle

• Liquid marrow is aspirated and spread thinly to assess cell morphology, maturation, cell proportions and iron stores.

Specimen

• Adults: posterior superior iliac crest is preferred; sternum (adults only) when needed. Infants: anterior tibia in very young children, or iliac crest.
• Aspirate only a small volume (about 0.2–0.5 mL) for smears to avoid dilution with blood; further samples in EDTA (flow cytometry, molecular) or heparin (cytogenetics) as required.
• A trephine biopsy is taken when a dry tap occurs or cellularity and architecture are needed.

Procedure

• Make films immediately before clotting, from particles selected on a slide or watch glass; squash (crush) preparations of particles are also used.
• Air dry, fix and stain with a Romanowsky stain; Perls' stain for iron. Special stains (MPO, Sudan black B, PAS, NSE) and flow cytometry as indicated.

Results

• Low power: cellularity of particles, megakaryocyte numbers, abnormal clumps or tumour cells.
• High power: differential count of at least 300–500 nucleated cells in cellular areas near particles.
• Typical adult values (local ranges apply): myeloid to erythroid ratio about 2:1 to 4:1; blasts under 5%; plasma cells under about 3–5%; lymphocytes about 10–15%.

Interpretation

• Blasts at least 20% (WHO/ICC thresholds vary by entity): acute leukaemia.
• Megaloblastic erythropoiesis: B12 or folate deficiency.
• Increased clonal plasma cells: myeloma. Hypocellular marrow: aplastic anaemia.

Quality control and pitfalls

• Haemodilution (no particles, blood-like differential) gives misleading results.
• Dry tap suggests fibrosis, heavy infiltration or technique problem; obtain trephine.
• Delay causes clotting and poor morphology.

Clinical use

• Unexplained cytopenias, suspected leukaemia, lymphoma staging, myeloma, storage diseases, fever of unknown origin (culture, granulomas) and iron assessment.