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PA17.1 | Aplastic Anaemia & Bone Marrow Examination — Part 4

Normal vs Aplastic Marrow — Estimating Cellularity

Marrow cellularity refers to the proportion of the marrow space occupied by haematopoietic cells (as opposed to fat cells).

Estimation rule:
An approximate formula is:
> Cellularity (%) ≈ 100 − patient's age (in years)

Examples:
- Newborn: ~100% cellular (nearly no fat)
- Age 30: ~70% cellular
- Age 60: ~40% cellular
- Age 80: ~20% cellular (physiological reduction)

This age adjustment is critical: a 70-year-old with 30% cellularity may be normal, whereas a 20-year-old with 30% cellularity is severely hypocellular.

Aplastic anaemia:
- Severe aplasia: <25% cellularity (or <50% with <30% residual haematopoietic cells in remaining areas)
- Remaining cells are mostly fat cells and lymphocytes; no blasts, no fibrosis
- The sinusoids may be prominent
- Haematopoietic precursors virtually absent

Normal marrow architecture on trephine:
- Haematopoietic islands interspersed with fat cells
- Myeloid:erythroid (M:E) ratio = 3:1 to 4:1 normally
- Megakaryocytes scattered, ~5–10 per low-power field
- No fibrosis on reticulin stain

Four-panel diagram showing bone marrow cellularity assessment using the 100-minus-age rule with normal age-related examples and aplastic anemia comparison. — **Panel A:** 20-year-old normal marrow: 80% expected cellularity with hematopoietic cells (blue) and fat spaces (yellow). **Panel B:** 50-year-old normal marrow: 50% expected cellularity showing age-related fat increase. **Panel C:** 70-year-old normal marrow: 30% expected cellularity with predominant fat spaces. **Panel D:** Aplastic anemia example: 30-year-old with severely reduced 15% cellularity versus 70% expected.

SELF-CHECK

A trephine biopsy is obtained from a 30-year-old woman with pancytopenia. The biopsy shows 15% cellularity with no blasts, no fibrosis, and no abnormal cells. Which of the following BEST describes this finding?

A. Normal marrow for age 30

B. Consistent with aplastic anaemia

C. Consistent with myelofibrosis

D. Consistent with packed leukaemic marrow

Reveal Answer

Answer: B. Consistent with aplastic anaemia

Using the 100-minus-age rule, a 30-year-old should have approximately 70% cellularity. At 15%, this marrow is severely hypocellular — consistent with aplastic anaemia, especially in the absence of fibrosis, blasts, or abnormal infiltrating cells. Myelofibrosis would show fibrosis on reticulin stain and a dry tap. Leukaemia would show packed blasts.