Bone Marrow Pathology
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Direct answer
Bone marrow examination — aspiration for cytology and a trephine biopsy for architecture — is the decisive test for unexplained cytopenias, suspected leukaemias, plasma cell disorders and infiltrative disease. The posterior iliac crest is the standard site (tibia in infants), cellularity follows the formula 100 minus age, and the myeloid-to-erythroid ratio sits between 2:1 and 4:1. The aspirate gives exquisite cellular detail — megaloblastic change, ring sideroblasts, LD bodies of kala-azar — while the trephine alone reveals fibrosis, aplasia and metastatic deposits, which is why both are drawn in one sitting. A dry tap, repeated honestly, is itself a diagnosis: myelofibrosis, hairy cell leukaemia or profound infiltration until proven otherwise.
What you must remember
- Numbers to quote: cellularity equals 100 minus age (about 50 per cent at 50 years); M:E ratio 2:1 to 4:1; posterior superior iliac spine is the site, tibial in children under one year.
- Dry tap causes: myelofibrosis, hairy cell leukaemia (its reticulin mesh and pooled cells resist aspiration), and marrow densely packed by tumour or leukaemia.
- Megaloblastic marrow: giant metamyelocytes and band forms, nuclear-cytoplasmic asynchrony in erythroid precursors, and numerous megakaryocytes with nuclear abnormalities.
- Iron and MDS: Perls' stain grades storage iron; ring sideroblasts (iron in mitochondrial rims around nuclei) define sideroblastic anaemia and figure in MDS classification.
- Metastatic deposits in children: neuroblastoma is classical (small round blue cells, often rosettes, with marrow replacing haematopoiesis); Ewing sarcoma, rhabdomyosarcoma and medulloblastoma follow.
- Storage cells: Gaucher disease shows crumpled-tissue-paper macrophages; Niemann-Pick shows foamy, vacuolated histiocytes.
- Indian infection pearl: Leishmania donovani amastigotes (LD bodies) inside macrophages appear on marrow aspirates in kala-azar, making the marrow a diagnostic site for a treatable infection.
Pancytopenia read off two preparations
A 40-year-old with fatigue has pancytopenia; marrow sampling proceeds from the same puncture. The aspirate smears show hypercellular fragments with giant metamyelocytes and megaloblasts — give vitamin B12 and the counts recover within weeks. Change the picture: the aspirate yields little, and the trephine shows streams of fibrosis with scattered megakaryocytes — primary myelofibrosis, whose teardrop cells were visible on the peripheral smear all along. A third version: the aspirate drips with plasma cells forming sheets over 10 per cent, monoclonal protein on serum electrophoresis — multiple myeloma. The identical presenting complaint resolves into three diseases by reading the two preparations together, which is exactly how FMGE frames it: a cytopenia, a marrow finding, a single best diagnosis.
Where students slip
The cellularity formula is quoted but then misapplied — a 60-year-old with 80 per cent cellularity is hypercellular, not "normal for the young", and a child with a fatty marrow is alarming, not reassuring. Candidates forget that aspirates cannot assess fibrosis or packing, calling a dry tap a failed procedure when it is a positive finding. Ring sideroblasts get confused with sideroblasts generally; the "ring" belongs to the perinuclear mitochondrial distribution, and iron stains make the diagnosis. Finally, do not report metastatic neuroblastoma cells as leukaemia blasts — their tendency to rosette and their clinical setting of an abdominal mass separate them.
Frequently asked questions
How is marrow cellularity estimated with age?
By the formula 100 minus age as a percentage, so a healthy 50-year-old marrow is roughly half cellular.
Which conditions produce a dry tap?
Myelofibrosis, hairy cell leukaemia and marrow densely infiltrated by tumour, all forcing reliance on the trephine biopsy.
What defines megaloblastic marrow morphology?
Nuclear-cytoplasmic asynchrony with giant metamyelocytes and megaloblasts — nuclei maturing slower than cytoplasm from B12 or folate deficiency.
What is a Gaucher cell?
A macrophage laden with glucocerebroside appearing as crumpled tissue paper, contrasting with the foamy vacuolated Niemann-Pick cell.
Why examine marrow in suspected kala-azar?
Leishmania donovani amastigotes (LD bodies) circulate within macrophages of the reticuloendothelial system, so aspirate smears can confirm the diagnosis directly.